The games have sold a ton already though. How many people are going to buy them again, just a few years later? I certainly wouldn't.
With that said, maybe I would be enticed to pick up some that I skipped.
Assumption I made here was PS5 will have backwards compatibility. These games would get the patch like PS4 games did for the Pro.
It think at the very least you should expect native 4k and a stable framerate on all games that support a PS4pro now. As for 60fps thats another matter.
A native 4K at 30 fps for PS4 games would be a minor improvement over PS4 Pro. Reports suggest PS5 will have 11 TF GPU and Ryzen 5 CPU which will be a massive boost over Pro, more than Pro was over base PS4 and it still ran base PS4 games which were 1080p 30 fps at 1800p and checkerboard 4K and some at 60 fps like Infamous Second Son. 4K 60 fps should be inevitable for current gen games.
and what incentive do developers have to go back to old games and create new 4k assets for them?
that's a ridiculous, unreasonable request (demand?) to have.
let them focus on new games
They are Sony first party developers. Sony has the incentive that it will make PS5 an enticing product to PS4 owners who already have these games and will be able to play them at higher resolutions and framerates which is important to many people.
It also makes it more enticing to PC gamers because PS5 will lack exclusives early gen and many of them don't have PS4 because these PS4 exclusives are 30 fps. 4K 60 fps for these exclusives would make many of them buy a PS5 moreso because they have been hearing that PS4 exclusives are amazing from players and reviewers and it will finally be playable at a standard they have set for their games to be played.