> Wayland is far more modular.

Not sure I agree here, assuming you mean "... than X11". With Wayland, you put your display code, input-handling code, compositor code, session-handling code, and window-management code all in the same process. (Though there is a Wayland protocol being worked on to allow moving the WM bits out-of-process.)

With X11, display and input-handling are in the X server, and all those other functions can be in other processes, communicating over standard interfaces.