Right, and nothing prevents you from writing a compositor that talks to an external process to handle window management. It just happens that no-one does it, be

Right, and nothing prevents you from writing a compositor that talks to an external process to handle window management. It just happens that no-one does it, because it's insane and difficult to make work properly. The compositor's job literally is to juggle the clients' buffers, which includes deciding what gets drawn on the screen and how. There are other processes involved though where the compositor and the client do talk the Wayland protocol via IPC, such that the compositor hands off various responsibilities to external processes. As far as I understand, X11 ended up with the window manager architecture pretty much by accident; initially, window managers didn't even exist.