Mir is the definition of NIH syndrome, i.e. an example of what not to do.

Mir had an important thing going for it: Much faster progress towards replacing X11 on the desktop with all its features. It took several years after the failure of Mir for Wayland to become really practical.

That said, I'm not sure if Mir would have been a good thing in the long term, mostly because of Canonical's control over it.

I agree.