I guess, this will not happen. The FPS genre is much to diverse. Look at games like HL2, Call of Duty, Bulletstorm, Borderlands and Bioshock. These are all FPS games, but very different in look and gameplay. The music games (guitar hero, just to name it) were all the same. More than 20 guitar hero and rock band games in 2 or 3 years, with no difference in gameplay was way to much.
FPS games are now around since wolfenstein and doom and games for core gamers . Since then a lot of different gameplay innovations happened. From that perspective i would say, that maybe the military shootes like Call of Duty or Battlefield will see a decline in sales, but there will be always successful titles like L4D, Halo or Duke Nukem (hope so). The FPS perspective in games is just a gameplay mechanic. No one would start a discussion, if 3rd Person Perspective games will die out.
