Rockstar has one of the best tech teams in the business along with massive resources. If they want to have a 60 fps mode on the base PS5, they can achieve that, although I’m sure there will be visible compromises.
there will only be a 60fps mode if they have good enough fallbacks for their RT effects. specifically the global illumination, which looks to be a core graphical feature of the game.
best case scenario would be that the Series S couldn't handle their RT GI and they had to have a fallback, which in return could mean a 60fps mode without raytracing is in the cards for the other consoles.
they aren't making a PC version at launch, so they do not have to keep mid range and low end PCs in mind in their original console launch version and can work on that issue after the console version is out, at which point they might not even need a fallback for their RT GI anymore either due to the PC market being saturated with cards that are good enough to run the game with RT GI.
so yeah. RT reflections can easily be replaced by SSR, so that's not an issue, shadows probably too as they already have a system for decent real time shadows in their engine.
imo it comes down to the question if the GI has a non-rt fallback, because that's way harder to replace and adjust