Second, what their customers are really buying is support. The ability to call Red Hat up on the phone, tell them about something that is broken, and have an actual developer go into the code and fix it for them. Red Hat can't really provide that with Mir, at least not to the same degree, because Mir is controlled by Canonical. They don't have the ability to help shape the direction of the project, or even have commit access to fix bugs.
If Wayland turns out to be a total and complete disaster, and Mir is a huge success, what you might see is Red Hat and others forking Mir into their own code that they could then run with however they want, but that seems extremely unlikely, for a lot of reasons. Including the GPL license and others. I'd say there's probably a better chance of them just giving up and returning to X full time than actually switching to Mir.