Well isn't PS5 just next gen PC parts essentially? PS4 is current gen PC parts, so in theory it won't be hard to make the games work. Just like games on steam work lol.
Not quite, since hardware and software design are much more specialized than PC, and even there you might have compatibility issues between operating systems and hardware configurations. The fact that they are both x86 isn't a guarantee that games will run going forward. It depends mainly on whether the CPU / GPU hardware architecture fully encompasses features that were present in former hardware. Say that the Navi GPU axes some features that existed on Polaris / Southern Islands GPUs. Then suddenly it becomes a much more complicated issue to be backwards compatible. Same for the CPU. That's why backwards compatibility needs to be a conscious decision on both hardware and software design. The PS3 for instance was an even more specialized design so there's little chance a hypothetical modern PowerPC + modern Nvidia GPU console could natively run the code (without emulation) that used to run in the Cell BE and the RSX.