With X11, it was simple: everybody used Xfree86 (or eventually the Xorg fork, but forks are not reimplementations) and libX11 (later libxcb was shimmed underneath with careful planning). The WM was bespoke, but it was small, nonintrusive, and out of the critical path, so bugs in it were neither numerous nor disastrous.
But today, with Wayland, there is no plan. And there is no limit to the bugs, which must get patched time and time again every time they are implemented.