> And 4:2:0 is just a poor fit for certain categories of images (like various kinds of illustrations, screenshots, etc).

I'd go even further and say that 4:2:0 is unsuitable for still images, period. Even for cases where it would be a suitable tradeoff for video. And even for video there's a significant chunk of content where it's undesirable.

In general I think we've come to accept some pretty gnarly limitations on image quality that really should not be such a given. 8 bits per channel is another one. You actually really do notice the banding with the naked eye in a fairly large range of situations, and if everything is 8 bits per channel then it gets really easy to exacerbate the problem on edit. Dithering in 8 bits per channel isn't really a solution because it's hard to do and it gets lost in compression quickly.

4:2:0 is the main reason why most games look like crud once they've passed through any mainstream video encoding pipeline. That being said, the bitrates used for streaming games and VoDs are so low everything looks like mush anyway and millions still watch that.