I guess Ubuntu's popularity may result in Mir winning out over Wayland? Or upstreams supporting Wayland primarily but most users actually using Canonical-patched Mir adaptations of upstreams? Things could get weird.
I use Linux and OSX desktops every day and catching up to OSX or Windows in terms of everyday usability via reasonable graphics performance is basically impossible without the ecosystem working together.
That being said, Ubuntu can do whatever they want, it's their money and resources and they can sink it into Mir if they choose but wow, what a waste.
Note that there's some debate on the accuracy of some of the examples. That they used it in the past or used it partly doesn't mean an entire company uses it now.