CRTC planes and scanout-time compositing makes sense, and Wayland compositors do use them, even for non-cursor surfaces. It's simply not something an application can use general-purpose and guarantee (though see the recent GtkSurfaceOffload stuff for the latest attempt at it).

Personally, I don't see it as a "2D drawing API", it doesn't accelerate anything special about 2D, only blits and transforms, which a 3D API will eat for breakfast.