Because for us (Xfce) that would probably take 15 years.

The only component that requires major changes is the WM. Otherwise, for the rest of them, it's more about fixing platform specific quirks.

(Ok, we did have to write an embedded compositor library for our panel to use, but... yeah.)

In this case, because XFCE is based on GTK, and the backend for GTK has gotten a lot of support for native Wayland recently. Starting from scratch would probably end up duplicating a lot more code than necessary, whereas supporting both in one codebase helps keep the featureset in lockstep for both x11 and Wayland users.

Why rewrite stable code? rewrite the layer that you need to rewrite. There are plenty of new DE for wayland if you're so inclined, however.