That's something I kinda wish more game ethusiasts would pick up on. There's not really a such thing as "4K" or "1080P" or whatever; it's more about fill rates and other numbers and the question of how much you can push at 1080P or whatever, vs. what the developer tries to push.

There's actually a couple of PS2 games that push 1080 (can't remember if it's P or i) with the right setup. But obviously, there's a quality stepdown.

There's no particular reason other than lack of hardware outputs the PS3-era machines couldn't push 4K games. They'd just have to be very, very simple games. I recall being impressed in the PS2 era by Katamari Damacy, which seemed to be one of the few games of its era that noticed that if you only allocate like 10 polygons and a teenytiny texture for a game world object you can have a crapton of things happening on the screen, at a high frame rate, and build a game around it. I can only imagine what a full-powered, brand-new Katamari Damacy would look like on even a Switch today.