"Everything is an extension" architecture is great when there's an incentive to promote extensions to some sort of core/standard library and then make them available everywhere.

If you look at the libxcb and xorgproto repositories you'll find something very similar to Wayland but for X (an RPC framework for building display servers / clients). The main difference is the X11 extensions are already known quantities thanks to X's monolithic history.

Developing a display server standard is a problem that Microsoft and Apple don't have to worry about.