One of the really brilliant things about a growing market is that next to no series need to die or decline. From last generation to this one, virtually no franchises decreased in size. The very rare exceptions are series that changed ownership and a few (but not many) Ps3 exclusives. The only multiplatform series that have properly decreased from last generation to this is Crash Bandicoot and Need For Speed.
Even Grand Theft Auto - which went from being the best selling game of the generation to just a large game - barely even dropped in sales, from 23 million with San Andreas to 20 million with GTA IV.
Games might lose market share in their genres, but very, very rarely do they significantly decrease in size. The only ones it's possible that will significantly drop are really Nintendo's games, because they've been so immense this generation, and if WiiU is just a moderate success (40-60 million), they simply can't sell 30 million. Still, they'll be selling over 20 million on the 3DS, and most likely will keep doing so on future Nintendo handehelds.
Whenever a series has a proper decline nowadays, it's because the entire genre is decreasing. We saw this with music games this generation, and we've seen it partially with Nintendogs-style games and traditional racers.
If Call of Duty or 2D Mario sees a giant reduction in sales, it's because the whole shooter genre or 2D platformer genre takes a major population hit.
