This is not about FOSS, it is about one man's vision to circumvent the mainstream developer community. Ubuntu chose to make the transition from pre-configuring Debian and making compiz themes for Gnome to trying its hand at systems-level programming. It would be somewhat self-assured of them to assume that all hardware vendors, who are already heavily invested in Linux, as a platform, would necessarily pick up the slack, especially when those investments did not foresee a move like Mir.
I don't think it's entirely a corporate move here. Free software contributions aren't infinite; if you have $N developers working on integrating support for display servers, then it's simple math that having to support two display servers means that you'll have $N/2 developers working on each, and will therefore get half the work done (and, since they'll probably still have to support X11 for a while, make that $N/3). Ubuntu selfishly forked Mir instead of choosing to try to use Wayland (especially when they had originally committed to using Wayland[1]), so the burden is on them here. It also doesn't help that Canonical employees were using the inclusion of XMir support as political fodder[2].
