I don't think gamers are getting dumber. I don't even think that developers think gamers are getting dumber. There's just so much money involved with high profile video games these days, that it's better to play it safe, go straight for the most unskilled and impatient gamer and level it off with optional things like difficulty settings later.
Remember playing turn-of-the-century FPSes like Soldier of Fortune 2, Medal of Honor: Allied Assault or Jedi Outcast and having to "use" every door to find out which one was unlocked? Back then, that was just something we were accustomed to. Then Call of Duty 1 started the "invisible director" trend, where your squadmates huddle around the door you're supposed to enter and just stand there until you do.
Developers have just become scared of leaving a game's flow and pacing in the hands of the gamer, or rather the gamer's skill and patience.