I'm sure Mir is a perfectly good replacement to crufty old x.org. But you know what? Upstart was a perfectly good replacement to crufty old sysvinit, and we all know how that one turned out. I predict this will turn out exactly the same; Ubuntu will go with Mir for a few years before eventually caving to community pressure and switching to Wayland, just like the ultimately gave up Upstart in favor of community-favored systemd.