Personally, I probably will return to TMU once the Canyon craze has died down a bit and it becomes a normal game to play rather than something that feels brand new. Even with a new game where most of the community is I'll still have a desire to build in something like Island or Coast every once in a while.
For the community as a whole, not everyone is going to like it. I'm pleasantly surprised a good amount of Nations users have made the switch, but there's still a lot of environment "specialists" that probably were never going to like Canyon because it's not like the one environment they decided to only play out of the 7 available.
And I'm not just talking nations users, but also people who only played Island, or Coast, or even Desert.
I think for the most part it will bring the communities of Nations and United together, but there's always some people from both communities who will choose to not make the switch unfortunately. Still, it seems the activity here is great and I'm glad I'm a part of the TM2 community now.