Except that X11 is NOT "mostly working" for a significant amount of modern usages.
Yes, if you happen to want to run your Terminal, Emacs, etc. remotely over a network connection, X11 is your huckleberry--but only because anything more complicated than a bitmap makes that a very difficult problem.
If you want HiDPI, subpixel anti-aliasing, color calibration, multiple resolutions on multiple displays, smooth video, no tearing, etc. X11 only works to a certain degree, some days, for some people, for some video cards. And it's not even clear how those should work over a network connection.
This is not new. After all, X11 critiques came in for a full chapter in the Unix Haters Handbook and that's 25 years old. Wayland is simply revisiting the same problems as X Server Extensions 30 years later (they aren't ubiquitous so nobody uses them so they aren't ubiquitous).
The problem is that people who use obscure features are VERY vocal about it while people who simply abandon your operating system because accelerated video doesn't work reliably are very quiet.
Maybe Wayland isn't the way forward, but X11 certainly hasn't been the way forward for decades.
The only advantage I can see is that if Wayland manages to tear out the X11-isms in everybody, what comes after Wayland will have a lot easier time of it.