It could never all be on one disc though because it wouldn't fit. You'd need one BluRay for the PS4 and one for PS5. If PS5 is UHD, that disc wouldn't even be read in PS4 would it? The only way you could have one disc is effectively one executable like PC, which would mean gimping the PS5 version for cost reasons to produce a game targeting a lower common denominator instead of developing for next-gen features from the ground up.
And if using two different executables, that's no different to any other generation. We could have had a CD and DVD PS1/PS2 combo, and a PS3/PS4 BRD combo.
If you want to lock in players using their existing hard-copy library, I think you'd just need an upgrade path. One thing you absolutely would not want to happen is devs to limit the next-gen experience unnecessarily due to supporting last-gen machines on the same game.