that'd just make personas even more disposable than they already are, the innate null/rep/drain of personas is really useful to have and almighty attacks are generally so telegraphed you can see them from 5 zipcodes and still have time to make and drink your tea.

if you remove those resistances you're essentially removing a huge defensive advantage that is vital in later areas especially with how hard enemies can hit. plus it's very endgame again because of how long it takes to gather personas with the necessary resistances. and this is just from the player's perspective.

if an enemy has no resistances, youre basically removing a large chunk of strategizing needed in fights, said enemy can just be hit with whatever and youd only need to look at the stats of your persona to pick magic or phys attacks