Depends on the game you're looking for.
Most of them, I'll say yes, but that's not necessarily a 'bad' thing.
Personally, there's always people, (ie: me) who will squeeze every last drop out of the game, with 500+ hour saves in Fallout 3, multiple replays and speed runs of Mirror's Edge until you have the routes down perfect, getting a mod to increase level cap to 999 in Torchlight and spending forever in the Shadow Vault, playing the multiplayer of an FPS that had a ~10 hour campaign for 400 hours, etc.
But for the average player, I would say, yes, the main story modes are getting shorter (Bioshock, Mass Effect, etc.), but for the average player, maybe it will seem like less daunting a task to get involved in a new game if you know that you have a chance to actually beat it before another one you wanted comes out.