a) that's a QA nightmare if you have to test every application with both X and Wayland backends.Also, most apps will use an abstraction like QT or GTK, so will work with any backends supported by those.
b) having to tell every application whether to use X or Wayland would be a pain.
c) numerous applications simply won't work on X unless they're written to use it; most obviously anything that performs video playback. Not that video playback to a remote X server makes much sense of course .
To me, when the IT hype is heading back to the centralised server, thin client idea, throwing away all the features that were designed into X precisely in order to support that concept the last time it was going to be the future seems freaking insane.