The problem is, X11 doesn’t really work fine for modern usage.

It kinda falls apart with multiple monitors, especially when they require different scaling or refresh rates (or both), HDR support would be incredibly difficult to add, it’s buggy, it’s virtually impossible to maintain or add features. Often fixing a bug breaks things, because the bugs in it are so old that programs have actually been designed around them, or even to utilise them.

Now imagine trying to adapt X for use with VR/AR displays and all the differences in window management that’ll be required for that.

It’s a security nightmare. Any app can see what any other app is doing. That means that if you have a nefarious app, it can scrape any information on your screen, without even needing root privileges. Then there’s a load of other vulnerabilities.

The developers have moved to Wayland because X is structurally unfixable.