Games ARE getting dumber, but not in sense you imply.
Lets look at pong, not exactly mentally stimulating. Compare that to crysis, you'll probably think a lot more than when playing pong. I chose the worst example here to make the general point. Games are dumber now because they hold your hand, they give you ad nauseum tutorials, half of most games are tutorials, including some of the best games in recent memory (portal is a giant tutorial for the final like 2 levels. insert rabid comments about how much the cake lies here) Remember Star Control II? That game did not hold your hand, if you did the wrong thing, OOPS, game over, and they would be happy to let you play for weeks without telling you that you had already lost. But now, try not holding peoples hands, and you'll have screaming and hate on interweb forums.
Moving ahead from pong several years, lets look at some immortal games:
1. Super Mario Bros.
2. Space Invaders
3. Galaga
4. Super Ghouls and Ghosts
1. Jump on things, minimal thinking, mostly skill.
2. Flying from the left to the write shooting an infinite number of ships. Timing and skill.
3. Same as above.
4. Absolutely the hardest game ever made, only skill.
But we chose the games we wanted to make our points about. If you want your shooters to be intellectual, good luck. Enjoy the few and far between you get-- your deus exes-- and get over it because its not going to happen very frequently. Proof? Dead Space as an intellectual story shooter? Shock horror at best. If you want highly intellectual or crazy long stories, theres places to look. Gal Civ. Civilization. Total War. Europa Universalis. Sins of a solar empire maybe. Baldur's Gate 2. But if in depth strategy, and tactics, and learning the entire dungeons and dragons rule book on the fly isn't what you want, and you want to use in depth strategy and tactics while executing perfectly timed boom headshots, you may be dissapointed yet, though theres options out there.
And one little pet peev: if you want a great story, seriously, read a book, because the best storyline games often don't compare to average movies. We are biased towards games because we are gamers, and overlook the glaring issues. Proof? I love resident evil. And beautiful dialog they have not.