We stopped where we are at because the eye cannot see more colors. The same goes for 4K displays. More resolution doesn't make sense. 4K pixels is more pixels than there are cones in your eyes.
Go look at any CIE chart. The human eye is far more sensitive than the standard sRGB colorspace. For that matter, it's impossible to represent the full perceptible colorspace with just three colors. And even if you use a wide gamut, 8 bits per channel is not enough to prevent banding. Particularly over the full brightness range the eye can perceive, which is considerable. That's why "HDR" screens are at least 10 bit nowadays. We didn't stop at all.
As for 4k displays - most of those cones are in the center of your FOV, and eye movement is part of human vision (saccades). That's why 4k VR headsets still look pixelated. Also, you don't just sit in front of your screen staring straight ahead like a zombie (at least, I don't). You move your head. You lean in. By your logic, the edges of a screen shouldn't even be in color!
It's like saying a keyboard with more than 10 keys doesn't make sense because we only have 10 fingers.
