I can't comment on everything but:
> Unity (abandoned)
You probably don't recall how bad Gnome 3 was initially, and how much better Unity was over Gnome 2. Sure it also had its issues in the beginning, but it took a really long time for Gnome 3 to catch up.
> snap (flatpak, done differently, not yet abandoned :).
I am pretty sure flatpak didn't exist when snap was announced. Or it was around the same time.
Given the above two, I'd place very little credence on the rest of your points. I agree with the Mir situation to some extent, but look at how long wayland has taken to ship.