I wouldn't say FPS's are dumbing down gamers. I'd say it's more likely a case of them not raising the standards of gamers. A gamer growing up in the age of Call of Duty might find a more complex game like Mass Effect or Half-Life to be daunting, but it isn't because Call of Duty made them dumb; it's because playing Call of Duty simply doesn't prepare you for playing games like Mass Effect or Half-Life. It's just not on the level of those games, and what's more, it doesn't try to be. Now, whether or not Call of Duty should be trying to emulate Mass Effect or Half-Life is another discussion altogether. The point is, if all you play is Call of Duty, going to anything else is going to induce a pretty massive dose of culture shock.
In any case, it seems like the gamers of that particular mindset are fairly young. As they get older and start broadening their horizons, it seems likely that they'll be able to move beyond Call of Duty onto more cerebral fare.