Genuinely what is the practical difference in this for 99% of users? They just want to play x game. Proton performance is pretty great, what else would be a problem for those people?

Also when it comes to breaking proton support (Which does happen) Valve + GloriousEggroll give you access to plenty of older and special versions. Surely that's better than rolling back entire software?

My game doesn't work -> I go to protonDB -> Users saying use X Proton Version or Y ProtonGE version -> I switch the layer used in steam

Hard to imagine a simpler process than that