R&C being only on PS5 was more of a PR stunt by Sony so that they could claim that "we believe in generations" before turning around and making GoW, Horizon and GT7 all cross-gen. They could easily make R&C for PS4 if they wanted to with downgraded graphics. It was probably a smart move to make it PS5 only as it fooled most people on these forums even.
The dimension/world jumping probably would have been rough to do on a PS4, albeit you probably could have dropped that feature if you really wanted it on PS4.
Question is basically how long should a studio support last gen, if its just a monetary issue then I would say until you see more such a small number of sold units for the older SKU that it is not worth doing the work for it. I doubt there will be a critical mass of PS5 for a bunch of years.