> If they were to make a shim layer that would let a wayland-only app show up on an X desktop, they would go a long way towards relieving this concern.
Weston has had, pretty much since day one, a really competent nested-under-X11 compositor backend. It works pretty much like Xephyr, in that your desktop is rooted rather than being window-by-window, but it works great.
If the clamour of people who genuinely need X11 and cannot live with anything else is so great, a solution for this will definitely emerge.