"So that means our software won’t run on Mir. Since our software is not shipped with Ubuntu anyways, that’s a non-issue." ~ Aaron Seigo
As I thought, there will be software that simply won't run on MIR (without some cost in time and effort). Will be interesting to see how this all sugars off. I'm not worried about it from a user's perspective. However, if I had to make a prediction, I expect that Ubuntu's move (if Ubuntu defaults to MIR) will marginalize the rest of the linux ecosystem (if the rest stubbornly refuse to follow suit). Then again, they're all largely irrelevant anyway (sorry Arch, Debian, Fedora, PCLOS, Redhat, etc...) Ubuntu is the only distro (and Canonical is the only company) that has a chance in the larger scheme of things. All the other distros just don't matter (and they're not in it for that reason anyway) so an extra layer of irrelevance shouldn't bother them (until and unless certain video drivers only work with certain display servers). That's when all the little wheels will start squeaking very loudly.