Also, there's another angle to this, how this is likely to hinder the adoption of Wayland. Say, in the future, we'll have distros that use Wayland, and then Ubuntu using Mir (which is likely, as I don't see anyone else adopting it, no one else is using Unity either). So then Wayland programs won't run on Mir, Mir programs won't run on Wayland, but X programs will run on both. So from application developers' point of view, the choice is simple - to get their program running on as many platforms as possible, it's best for them to stick with X.
So all those who keep whining "who cares if it's mir or wayland, we just want to get rid of X now!!!!1111" you should know that this Canonical's move is probably only slowing down the deprecation of X.