The Snappy/Flatpak situation is different because each system has its own advantages and disadvantages and Snappy didn't come years after Flatpak.
What makes you so sure Mir would not have some advantages? It seems impossible to say either way. For all we know it could offer benefits and choice to the ecosystem. Or die because users would reject it.
Open source has always been about diversity and choice. There is now an unfortunate trend of some commentators misusing words like 'community' to push their own agendas or trying to pass off personal opinion as a widely held fact. This basically challenges open source diversity.
systemd was resented by some users, but for developers it had so many advantages (especially stuff like logind) that a huge part of them just started happily using it.
As for Mir, I cannot think of a single discussion on any communications channel ever where a Non-Canonical developer thought it was a good idea. The GNOME people wouldn't even look at it. The KDE people openly stated that they hated it (http://www.mail-archive.com/ubuntu-devel@lists.ubuntu.com/ms... and https://blog.martin-graesslin.com/blog/2013/05/mir-in-kubunt...). No Non-Canonical developer ever added support for it to any UI toolkit. For me, that's very, very close to 'one voice'.