poor code quality has something to do with it, but is a bad reason to kill the whole protocol and ecosystem.
And now that whe have LLMs it's easier then ever to clean that up.
(and wayland has still not caught up after all those years)
The protocol was dead about 2-3 decades ago, no one actually did X11 for a longggg time, the only few real X11 apps were xeyes and some weird apps like that. No one did X11. Mutter, KWin, Picom, Compiz and so on did the painting, X11 just provided an insecure input and display protocol.