I hope so too.
Ideally, the PS5 would be backwards compatible with all Sony disc based games.
Realistically, it will probably be backwards compatible with digital versions of the games which you will have to re-buy (for PS1 through PS3) PS4 discs will probably work.
Maybe, just maybe, we will get lucky and meet halfway in the middle, where you can play backwards compatible digital versions, but they are free if you can insert the original disc in the system.