See, I don't think the games have been getting shorter. I definitely think they feel shorter, but I think it is just that we are quicker at finishing them.
Probably a bit of the problems comes from the fact that you have to finish them quickly to be ready for the next round of games. It used to be that you had months to play Mega Man because there was nothing else like it to compete for attention. Same goes for Doom and the like, as well.
Granted, there have been some exceptions. I don't think I could speed through FF6 or 7 to save my life. Of course.... that doesn't work because I couldn't speed through 10, either. So... exceptions...
The FPS market, I will confess I don't know too much about. I didn't like Half Life when it came out. I was stuck in the multiplayer trap that was Quake 2. Then multiplayer got all involved and it took reading strategy guides to know which gun to buy. I didn't care for that. But all of the other genres of game? I wouldn't be too terribly surprised to find out games are actually getting longer.
Now.... I do believe games are getting too easy. I swear to all that is holy that I am sick and tired of the stupid "tutorial" crap that begins most games. Just let me figure it out. It can progress if you want, but DO NOT INTERRUPT me. (I just started Metal Gear Acid. That series is notorios for this. "Snake, use x to climb the ladder in front of you." "Ok, now press circle to examine the wall." )