> When you can tell Xrender to efficiently composite some pixmaps then there's really no reason to use sub-windows ever.

There is, or we would not be having subsurfaces on Wayland or this entire discussion in the first place.

Are you seriously arguing that the only reason to using windows in Xorg is to have composition? People were using Xshape/Xmisc and the like to handle the lack of alpha channels in the core protocol? This is not what I remember. I would be surprised if Xshape even worked on non-top level windows. heck, even MOTIF had windowless widgets (called gadgets iirc), and the purpose most definitely was not composition-related.