The story that X11 somehow forces programs to use outdated drawing primitives was never really true as a long as I can remember. Compositing on X11 exists.

In principle, it is an extensible generic remote buffer management framework. As such, it could be extended indefinitely without every breaking backwards and forwards compatibility and is also a good fit for modern hardware that essentially deals with remote buffers on the GPU. As someone doing HPC programming on GPU, network transparency and GPU are definitely not at all allergic.