What I don’t get is why they pushed the compositor onto WM developers in the first place. Compositing seems like way too low level of a task for a window manager to be concerned with.

Do you mean default compositor?

I think they meant like a reference implementation, wlroots came a decade into Wayland

I mean they should have made Weston production grade and extensible so that others can integrate with it instead of remaking their own compositors.