Why can't GG just put more animations, physics and AI routines into the PS5 version, while balancing which of those elements can be implemented into the PS4 version? If you think it would make the game play too differently, then it's mostly an issue of AI and physics. AI in games has hardly been computationally-bound. Games don't have genius-level AI, and it's not because the CPU can't crunch the numbers. So that leaves physics. You can very much have a more complex physics model in the PS5 version versus the PS4 version. That's something PC games have done ever since the introduction of PhysX.
Here's a thought, how does R&C demonstrate animation, physics, and AI beyond what a PS4 can do? That's a PS5-only game, but everything we've seen of it is PS3 R&C with shinier graphics, and world warping. That's plenty next-gen enough for me, and seemingly for everyone else. So why are we pretending that there's more to it than that?
tl;dr: The best showcase of the PS5's ability so far looks like a game that could be played on the PS3, if it had an SSD to handle the world warping. The map loading is what sets R&C apart from what could be done gameplay wise on a PS3. Other than that, it's exactly what we should expect from both cross-gen and next-gen games, a bump in resolution and detail. Demon Souls is PS5-only, and that is just the PS3 game with shinier graphics. GT8 will probably be PS5-only, and that will be GT4 with shinier graphics. The next Uncharted will be UC2 with shinier graphics. Should I continue? There's absolutely nothing wrong with next-gen being a new coat of paint. That's exactly what next-gen has been for decades now.