So you've beat the solo campaign then and got all the author medals?
I think that'll take me about 3 years to do. Well, the author medals will.
And just 'cause you play Canyon doesn't mean you can't enjoy other games, including the old TMs.

They're "Forever" anyway.
But, to answer the topics question, since I never have yet... and I've posted in the topic a few times...
Yes, the game will definitely hold players for 9 months. Heck, it'll hold a lot of players for several years. And yet, new environments will be out before that. Look at all the people who still play TMO and focused on only desert, or TMN/TMNF players who have played a single environment for years. Don't give me that crap about stadium that it's only because it's special. It's still a single environment. Or even within TMUF, people focus on a single environment, like Coast... or Island. Probably over half the TMUF community specializes in one of those two environments.
Canyon feels like a "greatest hits" environment to me. I get so many different feelings when driving on the different road surfaces; GP, Platform, Roads, Canyon Roads, Offroad. The physics are built for maximum fun. And there's so much variety of tracks already... and some really amazing and fun hunters out there.
The game will be as entertaining and fun as the community makes it. If you leave, you're just contributing to it going the wrong direction.
Stay and be part of the reason to stay.
I had left the TMUF community after 3 years for personal reasons and wanting to focus my time elsewhere. The game never got boring, but I just needed to reclaim some time... and TM was taking a lot of it.
But I've found I can still fit some TM into my life. It's so much a part of who I am.
So I won't be as active as I was in TMUF, 'cause priorities in life won't allow it. But I'm sure I'll play it regularly and I'll enjoy it for a long time, certainly more than 9 months.