Writing a Wayland compositor on top of wlroots is probably a larger project than writing a window manager for X. But it's not orders of magnitude, wlroots handles a whole lot of the dirty work for you.

> The X protocol [...] doesn't spoil like cabbage.

It actually does. The protocol doesn't handle things we expect from modern hardware and software. And the implementation certainly rots away as the rest of the software ecosystem changes in ways which will slowly make Xorg more and more broken. Drivers will change in ways which expose Xorg bugs. GUI toolkits will introduce bugs in their X backends and introduce new features which don't work on their X backend. Nobody maintains X anymore, and something like a display server in a rapidly changing software and hardware ecosystem requires maintenance to keep functioning.

None of this is to say the Xmonad developer should spend time rewriting their project to use wlroots instead of X. But as long as it requires X, it's living om borrowed time.