It will be interesting to see what happens in a post COVID world.
Having every line at DW be Standby isn't going to cut it and we all know it. I don't believe MaxPass is the best option at DW and I can explain why if pressed.
If the FP+ system is getting replaced, I envision a reservation system that goes in tiers, with paid up-charged Tiers. Something along these lines (no pun attended):
Tier Base = a Reservation for one of the parks, two FP (from non headliners), and a meet and greet FP. You don't get to select the time, although indicate a preferred time.
Tier Plus = a Reservation from one of the parks, two headliners FP and a meet and greet FP. You don't get to select the preferred time of the attractions (Early AM, Late AM, Early PM, Late PM)
Tier Max = a Reservation from one of the parks, 3 FP (any of them you choose). You also get to choose the preferred time.
Plus and Max would have up-charged costs.
This system still allows Disney to control crowds through reservations, as well as an effective and profitable way to distribute FP. All tiers could "sell out", again this controls FP distribution. Disney also wouldn't have to redesign all the FP queues, since they would still be in usage.
Day of FP will be still be available once your initial 3 are consumed.