Excellent analysis but I'm going to disagree a bit... in terms of "equivalence" and the categories currently covered by each.
For example, in functional use by photographers, the D850 and A7riii are definitely equivalent, with users preferring 1 over the other for whatever reasons. (valuing the small size, live view, etc, of the A7riii... valuing the deeper grip and better weather sealing of the D850, etc)
We can roughly break things down into 3 categories that the camera makers use to differentiate their cameras:
Image quality
Performance
Build
We can roughly break those 3 down further into low/medium/high
Image quality:
Low -- the ubiquitous 24mp-ish aps-c sensors
Medium -- the 24mp-ish full frame sensors
High -- high resolution full frame sensors
Performance: This is the quality of the AF system, the burst rate, the buffer depth... etc
Low: think Canon T7 -- 9 AK points, 3 fps, 6 image buffer
Medium: more advanced AF systems, 7-10 fps systems, with adequate buffers for casual sports but not necessarily hardcore..
High: 12-20 fps, flagship af systems, buffers made for hard core sports professions
Body:
Low: Plastic, light weight, small viewfinders, in mirrorless -- low resolution EVFs with slow refresh, or no EVF, no weather sealing, often smaller batteries and poor battery life
Medium: good access to manual controls, weather sealing, higher quality VF's, etc
High: Top quality VFs, often with battery grips, best weather sealing, best connectivity, dual card slots with faster media cards
Now, in the body especially.... there are a lot of in between levels.. there are lots of mid/high... low/mid... Especially when you throw in mirrorless which just somewhat prioritizes different things about the body. And of course, there is a lot of subjectivity in body preferences-- different people just like the feel of different cameras. But there are still objective measures as well.
Now, if we look at the dSLRs and mirrorless equivalents:
Truly low:
Low IQ + Low performance + low body
Canon T7 and Canon M100
Nikon D3500 and no current mirrorless
Sony A5100 and maybe A6000 at this point
Next tier.. here, we mix it up a bit.. They are all still "low IQ" -- 24mp aps-c sensors.. but there is variety in build and performance
Generally: Low IQ + medium body + medium performance
Canon Rebels, Canon 80D --> M50 (and M5.. but it's kinda an odd duck, surpassed by the cheaper bodel)
Nikon D7500 --> no mirrorless yet
Sony A6000, maybe A6300/6400/6500
The Sony's may be in their own class... at least the A6400/A6500: Low IQ, low-mid body and HIGH performance... but that brings me to..
The typical high tier of aps-c:
Low IQ (still 24mp aps-c), high performance + high body
Canon 7dii --> no mirrorless equivalent.. but even the dSLR is old.. are they ending this type of camera?
Nikon D500 --> no mirrorless
The Sony A6400 and A6500 fit in this category in terms of performance, but not in terms of build.
Now, we have far more FF categories as the market had shifted to FF:
Full frame starts differently, there really aren't any completely low end bodies in full frame.. the camera makers always stick at least a half-decent body on their FF cameras.
So the basic full frame:
Mid IQ (24mp FF sensor, more or less), medium body, low performance
Canon 6Dii ---> Canon Rp
Nikon D610, maybe D750 (just because of their age at this point) ---> No mirrorless equivalent
Sony A7ii (filling this category with older models)
Next we get the plethura of $2000-$2500 FF cameras... they are basically all:
Mid IQ (still 24mp-ish sensors), medium body, medium performance
Canon -- the 5Div is at the high end of this category..... the Canon R is the "equivalent" but it's more the low end of this category. They are aimed at a similar market, the 5Div is just a step above
Nikon -- in dSLR, this market is currently empty, but the D750 sat in this market (with the progress of technology... it was "medium performance" in 2014 but today would be considered lower than average performance, with an aged AF system, terrible live view AF, 6.5 fps is no longer above average). In mirrorless, this is the Z6.
Sony: A7iii basically defines the category
Now things get different...
High IQ, medium body, medium performance:
Canon -- Nothing in this space currently. They have the 5Ds models, but those are low performance.
Nikon: D850 (but both body and performance would rate more as mid/high than dead middle). ----> Z7. Yes, Nikon intends for them to be near-equivalent. The D850 is better in most measurable ways: dual card slots, better AF system... but the Z7 has some advantages including smaller size and better video features. They are 2 options with different pros and cons, but filling a similar segment. Partially, it's just that Nikon's mirrorless technology is still not fully mature.
Sony A7riii -- High IQ, medium body (large high quality EVF, dual card slots, uhs-ii support, weather sealed even if not as well sealed as some others), lots of direct control functionality (even if not designed the best). And medium/high performance (10 fps, excellent AF system, etc)
For the most part, any of these cameras can be, and are, used by professionals. Not sports pros -- But wedding pros, landscape pros, portrait pros, etc.
Then we get into what has traditionally be defined as the "sports flagship"
This is typically medium IQ (not the priority for sports flasgships), HIGH performance and HIGH end body
Canon 1Dxii -- No mirrorless equivalent
Nikon D5 -- No mirrorless equivalent
Then the Sony A9 with footnotes: It generally nails or surpasses in "performance" areas (ahead in some areas, behind in a couple, but overall equivalent)
Body.... harder to assess in the same terms, there is a design choice to go smaller, and not have a battery grip built in. Has the dual card slots, high quality VF -- the only blackout free VF. It has some advanced connectivity like FTP but not as much connectivity as the traditional flagships, doesn't have illuminated buttons. And in a real error, only has 1 fast media slot with the other media slot being slower. It's not a low end body... not a medium end body. I would call it a high end body, but not nearly as high as the 1Dxii and 5D. Like comparing a multi-millionaire to a billionaire -- they are both rich but the billionaire is in a different stratosphere. (which is also why the A9 is half the price of those other models).
But my buying advice would depend totally on how one intends to use the camera, as well as whether they need to buy now or wait.
If you're already invested in a mount, there is rarely a reason to switch.
For those that are really mount agnostic, no investment in any mount...
If you're true entry level looking for the cheapest camera -- doesn't really matter whether you go dSLR or mirrorless, and there are lots of options
For those medium enthusiasts, who want a capable camera beyond the barebones, but don't want to spend thousands.... I'd tell these people to skip Canon and Nikon for now. It doesn't make sense to start investing in a dSLR mount right now. The future of the Canon M is kinda unknown. Nikon's mirrorless aps-c future is unknown. This is the key market for some of the Sony and Fuji cameras right now.
For the really high end aps-c.... apc-s wildlife shooters and sports shooters... The Nikon D500 is still an excellent contender and there may not be a Nikon mirrorless equivalent for a while. Canon users should probably wait.. the 7Dii is old and there is no mirrorless equivalent. Sony users may want to wait to see if the A9 and A6400 AF system gets put into a better aps-c body. So this category is just basically under-served right now.
Casual consumer FF: Consumers with budgets that can get into FF.... lots of options and you can't go wrong. Can stick to traditional dSLRs like the 6Dii, D750, can go mirrorless with the Canon Rp, Sony A7ii, or expand budget into the A7iii/Nikon Z6/Canon R
For prosumer cameras... that can handle demanding amateurs but also most types of professional shooting....
Here there are lots of options but tradeoffs -- the D850 excels in this category, but the consumer has to ask themself if they want to invest in dSLR tech as we transition. Z6 and Z7 are options and would work for some types of shooters, but without dual card slots, with a somewhat lesser AF system, may not be up to the demands of this type of photographer. But, it will likely be 2+ more years before those cameras get an upgrade.
Canon users could be perfectly content with the 5Div but again, is now the time to invest in dSLR tech? and lots of issues with the Canon R and Rp make it difficult to really take those cameras too seriously in this category.
The Sony A7iii, A7riii and A9 can all fulfill these users right now.
And the sports flagship market...... here things get...... different...
The sports pro market will be the last to switch to mirrorless.
There is no reason to wait for Nikon/Canon to do sports mirrorless cameras -- because all the pro-sports lenses will continue to be dSLR lenses for the foreseeable future. Eventually that will change, but not for at least 3-5 years, minimum.
The A9 is indeed an option for this market -- but with caveats that it's a different style of body than such users are used to, and the native sports lens options are limited (a great 400/2.8, teleconverter options, a variable aperture but excellent 100-400, and a 70-200/2.8). So those might be enough lenses for many sports shooters, but fewer options than they can get in other mounts.
So right now... a buyer is in limbo in many ways.
Fuji does some great mirrorless cameras -- but they are aps-c only (and medium format).
Canon and Nikon are starting their transition to mirrorless.... but the aps-c mirrorless plans are currently unclear/unfilled. The full frame lineup has gaps, and the filled mirrorless slots have performance issues compared to their dslr models.
Sony is "mirrorless ready" -- sure, like all cameras, they have some cons. But they have mature AF systems, they have all the slots basically filled. You don't need to "wait for a Sony with good autofocus" --
So if someone was totally mount agnostic, I might recommend 1 of the Sony cameras.... but if they are already invested in F-mount or EF-mount... won't typically make sense to change.
Well, maybe, but Nikon's been making mirrorless since 2011, and Canon since 2012. Most people seem to forget that Z and R are their second mirrorless lens mounts so far, and the EF-M has been quietly taking over the Rebel space for some time.
Besides knowing that Nikon and Canon both micromanage their product lines to an extreme degree, and have had full frame mirrorless in the works for some time, I find it very interesting which cameras have been released so far. It shows how they themselves view the cameras, and gives us a good idea what's coming down the pipeline.
To back things up a bit though, historically, Nikon has made 3 cameras and Canon has made 3 cameras. Sure, there were lots of models, but there have only been that many body designs for quite a while. Skipping past the 1980's where things got really confusing, Canon and Nikon hired actual product designers to sort out their product lines and interfaces, and it became simple: Nikon made the pro bodies (F5, F100), prosumer (N80) and low level consumer (N55-N75), each one with a different interface, and that has continued on to this day. Canon did almost exactly the same thing with a darn similar design philosophy, with the Rebel/KISS, Prosumer, then true professional EOS-1. Across Canikon, you can see design similarities between each level - they finally found where the spaghetti stuck feature-wise and keep it up to this day.
Minolta, who sold far fewer cameras, only made 2 bodies: the 5 consumer camera, and the 7 and 9 share much the same prosumer design and interface. And do you remember the old joke about how do you double the amount of metal in a Minolta camera? Load the film. At this point, Minolta (and now Sonly) made/make no cameras similar to the F5 or EOS-1. Any camera like the A9 where I can make the lens to go out of alignment by holding it wrong isn't a professional camera, IMHO. Technical tour de force though it is, the build quality and user experience is prosumer at best by Nikon/Canon standards; though fortunately, its price reflects this. Sony's lineup is very much like the old Minoltas in this regard.
So when we look at the recent and current mirrorless and DSLR products and line them up based on the manufacturer's suggested use case, where are the missing products, then? Ideally you'd have an equivalent product across both mirrorless and DSLR - Sony does this very nicely at the moment, and I'm sure that's where Canikon are going.
For Nikon:
- No crop sensor mirrorless at all, so D3500, D5600 (consumer), D7500 (prosumer) and D500 (professional) stand without a mirrorless model.
- The Z6 prosumer body equates nicely to the D610 and D750
- The Z7 prosumer body has no DSLR equivalent.
- The D850 and D5 (professional) have no mirrorless equivalents.
For Canon:
- The Rebel/KISS lineup is equivalent to the EOS-M lineup.
- The 80D and 7D (prosumer crop) have no mirrorless equivalent
- The EOS R and EOS RP line up to the 6D
- The 5D (prosumer full frame) and 1D (professional) have no mirrorless equivalent
By a quick count, that's 4 Canon bodies and 6 Nikon bodies with no direct mirrorless equivalent, and 1 Nikon mirrorless without a DSLR equivalent. That's a heck of a lot of holes to plug in their lineup, but notice that there are no true professional mirrorless bodies from anybody, and no consumer bodies from Nikon. And there's a serious lack of good glass as well that will take a long time to fill, with some lenses from both that are over 30 years old still in current production that will need to be updated. Those are big holes that need to be plugged, but I expect not to see a true professional camera from Canikon until at least 2022 at the earliest, and more likely 2023 for the Olympics as per tradition, and even then the lens lineup won't be fully filled out.
And what about the fourth SLR mount, the Pentax K? Well, do you like pancakes?