The answer, of course, is that most of the community goes with it. In the age of the "annual franchise roll-over", when the next installment of Call of Duty, or Halo, or NCAA Football, or (insert game title here) rolls out many in the community have pre-ordered and are all ready to jump into the new game lobbies. However, there is still a substantial online gaming community left in the nearly-obsolete title's lobbies that want to continue to play. I am a Battlefield Bad-Company 2 (BFBC2) gamer, and I love the franchise. (This does not make me an EA fan, just a disclaimer) However, it has been almost 2 years since the game hit the stores and in that time Activision's put out two new
CoD titles. Since both are war-game FPS's, the community tends to vacillate and the BFBC2 community has dwindled.
Jumping on a server today, I'm lucky to find a game that has a full 12v12 population, the max that each map can accommodate. Also, when I do there are two things that happen: the teams are horribly mismatched because you've got the community that has been there since the beginning and is somewhere in the ninja-commando echelon of play and you've got the people that picked it up because it was $30 On Demand, were unprepared for the game's steep learning curve, and recreate the film Hamburger Hill at each engagement. When that happens the weaker team generally loses between half and all of its players as they quit to find an easier opponent, or a different game. The value of the game declines as the size and quality of its community declines, because it's main focus is multiplayer. The same could be said of some of the older CoD communities. Playing CoD: World at War (two titles back, now), there are the salty old bastards that refused to play the later titles (all entirely dev'ed by Treyarch) and you've got the On Demanders who picked it up at bargain bin prices. Again, horribly outmatched, dropping the quality of play, and fundamentally decreasing the game's value.