40 years old is also what makes it so hard to replace or even reimplement. The bugs are all decade old features, everything is written specifically for Xorg, all of which needs to be emulated correctly. It sure did serve us well, it's impressive how long we've managed to make it work with technology well beyond the imagination of the engineers in the 80s.

There's this for the protocols: https://github.com/probonopd/wayland-x11-compat-protocols

It can be done, it's just nobody wants to do it. It's not really worth the effort, when you can work on making it work properly in Wayland instead. That way you don't need XWayland in the first place, but also XWayland can then implement it using the same public API everyone else does so it works on every compositor.