They wouldn't have to redo the games. Games are made with high resolution assets which are simplified for the final release due to hardware constraints, even on PC. They would just have to bring their higher resolution assets in a patch.
By redo I meant the asset switching/ upgrading, not actual changing of the core code. But yes I agree it is feasible, but that it depends on easy it is for them to do. it should be, but you never know until it is done.
Sony have profited a lot from cross-gen sales for games that first were just PS3 launch. So I really expect some of the best seller will receive a vamp-up for PS5 and get additional sales even if BC exists. Now, only patch already sold games that person won't need to buy another game I don't think it will happen. At most patch to 60fps.
Reselling of games will most likely happen with 'goty' editions. Like the LToU remaster that included the DLC included. So re-releases of these particular PS4 games (that do not have an SKU with all content together) I think will definitely happen. Otherwise the games will just be patched. Reselling the same game with the patch makes little sense when you can just get a cheaper PS4 copy with a patch.
I think it will be great to get 60 FPS/4K for many of the classic PS4 games. I also hope to see some remakes of PS3 era games like the original Uncharted. Remasters are ok but full remakes with new assets would be so much better.
Nah, I am not sure why anyone would want remakes of PS3 games tbqh. Of PSP or Vita games, sure. UC Golden Abyss redone on the Lost Legacy engine (at the very least) with changes made to setpieces etc to make a better overall experience, sounds a lot more exciting to me than just redoing UC1-3.
So I would prefer the resources/ money was spent remaking pre-PS3 games or their handheld games, or just making new games.