> Nothing stops you from making a pluggable compositor. Or even creating a standard compositor protocol for screensavers and youtube-cat-video-viewer.
Sure. And that's the point. Wayland is advertised as a simple replacement for X.Org, that is supposed to be smaller, faster and easier to use. But currently it's smaller than X.Org for same reasons as Paint is smaller than Photoshop — it lacks many features. If you add all the missing features into wayland protocol it will grow as large as X11 is now.
Turning those features into plugins won't help much as well. It's just instead of large and complex wayland protocol you'll get a simple protocol together with large and complex weston Plugins API.
What's the point of wayland then? Reinventing X.Org from scratch?