X11 may "run really well these days" if your reference is X11 twenty years ago, but if you compare X11 to e.g. macOS, it's pretty obvious why Wayland is needed.

X11 may "run really well these days" if your reference is X11 twenty years ago, but if you compare X11 to e.g. macOS, it's pretty obvious why Wayland is needed. X11 network transparency was nice feature when 10 mbit ethernet was hot as pizza, but these days remote desktop protocols offer more practical alternative.