Sony initially claimed that they "believed in generations" and claimed that only making the game for the next gen would allow them to take the full usage of these new systems.
The thing with games is that developers have a lot of options available to them to port a game for a weaker system. Just look at all the games that got ported to the switch even though it wasn't originally made to be able to run at that hardware.
People who are still using a PS4 or xbone are probably fine with a game struggling to reach 30fps, these are not the type of people who buy a 3090 so they can get over 100fps in control with raytracing on (with DLSS). At a lower framerate there isn't the same need for a good CPU.
Furthermore the PS4 and xbone CPU has 8 'cores' (with each pair of cores sharing a lot of resources) so it can actually deliver decent performance when all the cores are fully used, the main issue has been that a lot of games are heavily single-threaded taking only good advantage of maybe 2 cores.
The main issue with the old consoles might actually be the slow storage (especially for people who haven't installed an SSD yet). But that does not mean porting will be impossible, you can just add loading screens or simply not fully load assets.
You can also modify the game (such as reducing the crowd-density in cities) to make it run better on weak CPUs.
The thing with games is that developers have a lot of options available to them to port a game for a weaker system. Just look at all the games that got ported to the switch even though it wasn't originally made to be able to run at that hardware.
People who are still using a PS4 or xbone are probably fine with a game struggling to reach 30fps, these are not the type of people who buy a 3090 so they can get over 100fps in control with raytracing on (with DLSS). At a lower framerate there isn't the same need for a good CPU.
Furthermore the PS4 and xbone CPU has 8 'cores' (with each pair of cores sharing a lot of resources) so it can actually deliver decent performance when all the cores are fully used, the main issue has been that a lot of games are heavily single-threaded taking only good advantage of maybe 2 cores.
The main issue with the old consoles might actually be the slow storage (especially for people who haven't installed an SSD yet). But that does not mean porting will be impossible, you can just add loading screens or simply not fully load assets.
You can also modify the game (such as reducing the crowd-density in cities) to make it run better on weak CPUs.