Judging by how HUGE this game is looking to be, I imagine the user BASE is going to be quite large and therefore carry some longevity with it. I always said that if more people PLAYED Twisted Metal from the beginning, it would have lasted longer. A year later, TM's online scene still has some life in it, but barely. God of War's MP seems to have an equal if unique fun factor that I think is only going to keep a large portion of its large user base playing for a long time.
As long as there is support for it from SSM, I think the online scene will be fine (matchmaking and variety questions still outstanding).