A couple of things to pull apart.
In strength training, what is this decision to change weights based on? So you're proposing that in strength training you shouldn't use the same weights (pace in running), same reps, and same sets over the course of a "training plan" and expect the same gains. I would agree, and that holds for running as well. Daniels (and other coaches) in particular adjusts all three of these variables over the course of a training plan. The # of reps and sets is adjusted within a 4-6 week block of time for a single workout or 14-18 weeks in a training plan. Just like in weight training you'd be ill prepared for the Week 6 workout if you tried to do it in Week 1. If you could easily complete the Week 6 workout in Week 1, in the context of the totality of the training plan, then it means something about the plan isn't the right fit for you. But Daniels also suggests changing the pace (or weights in strength training) as well. I believe the recommendation is to re-evaluate the pace scheme every 6-8 weeks, and no sooner than 8 weeks from the goal race. This allows you to move to faster paces if that decision is warranted based on performance, but also not to change too close to race day such that you don't reap the full benefits of that change. So Daniels, in particular, would suggest changing all three of those variables (as well as pace/duration of rest) as a means to leading to an stronger version of yourself at the end of the training. The key is making sure those workouts are quality.
So what's the purpose of Daniels R pace (usually around someone's mile pace, but sometimes faster/slower)? It's neurological and form based. You're attempting by doing really fast paced training to create a mind body connection centered around good form. "Where's the foot fall? How quick is the foot coming off the ground? Are my eyes up? What are my arms doing?" Does that solely happen at mile pace? What about 1500m pace? Or 2 mile pace? There does come a point where mile race pace becomes more sprint pace if you were to dip down into 100-200m areas. But ultimately, the pace isn't super tight. Rather the goal is to maintain good form. So when I schedule R pace in particular, I say to try and stay in a pace window, but if you can be consistent and maintain good form, then I'm fine with faster as that still meets the goal of the workout. But if form suffers, or if you become inconsistent with your pace splits, then the workout is done. So that might mean you were scheduled for 10 x 1.5 min at R pace, but by the 7th rep your pace has fallen off and form is suffering, then it's time for the workout to end. No value in pushing through on this particular type of workout (that's for a different day and a different type). So my advice on R pace, is to go as fast as you can be consistent with and maintain good form without moving your workout into a sprint.
So T pace is different. For Jack Daniels, T pace represents the Lactate Threshold pace. The pace at which you will accumulate more lactate in the blood than can be processed, and thus you see your blood levels go from baseline to a sloped increased value. The goal of the T pace workouts is to work in this narrow range for the expressed purpose of trying to improve your body's ability to manage lactate. So go too slow, and you won't reap the same full benefits. Go too fast, and you'll start cresting past the point of the lactate threshold and you'll accumulate in the blood. The pace faster than Lactate Threshold has been coined the "Critical Velocity" and has it's own set of workout design parameters for optimal outcomes. CV is a good pace to work on lactate buffering. So that's why LT workouts are best done at current fitness levels.
Now for you and me, we're going based on a calculator and other race based information. It's general for us and non-individual specific. The elites commonly use a blood lactate meter (something Canova does frequently with his runners) to truly track the bodies response to the actual paces being run on that day in that workout. Additionally, the lactate threshold is not a static value day to day and can fluctuate based on other factors (like fatigue and temp). So even if you had access to a blood lactate test locally, it really only serves it's purpose on that day and the data would be less relevant a few weeks later (from improvements and changes in other factors). So for you and me, we get a general LT pace and use that to try and get close to the goal workout outcomes.
Very rarely do I schedule workouts that push beyond what someone can currently handle. More commonly I use races for that purpose, because then they have a scheduled build up or down to safely handle the stresses. I don't find a good use for taking workouts to failure, unless that failure is prior to the scheduled end goal (like in the above made it to the 7th R rep and called it over before the 10th). That's not to say that other coaches don't have that mindset. For me, I take the long term approach. If someone can be consistent day over day, week over week, month over month, and year over year, but not put too much stress on the body at any specific time, then I think that person will be better off several years down the line. The person who does too much (beyond the current capabilities) more often sees setbacks. So they'll see the short term gains, and probably out gain the long term person in the short term, but then when aggressive person has a setback/injury, it'll put them behind at a later time compared to the long term (current fitness) person. Does it happen that way every time? Nope. Sometimes the aggressive person can handle it and sees a greater gain in the same time period. But I would guess that the long term person wins out in the end more often than not in a general population sense. A little bit of risk/reward in terms of choosing how you want to progress and where you see your personal goals 6 months, 2 year, 7 years down the line.
