I agree. Rejecting 2D as "old-fashioned" even when it makes perfect sense is just silly. I recently completed both New Super Mario Bros Wii (2D) and Super Mario Galaxy (3D). I'd say these are two different gameplay styles that are distinct enough to resist lumping them together in any single category of game (i.e., platformers). I enjoyed both, and surprisingly found Galaxy much easier (though many have complained about the great difficulty of NSBW). Nevertheless, the controls felt tighter and the challenges much more obvious in NSBW than Galaxy. I often felt in Galaxy that the difficulty was struggling with the camera and controls, whereas in NSBW the controls and camera felt tight. I also think a 2D game allows you focus attention better on precise jumping/timing sequences, whereas the 3D tends to draw your attention to the game world. When playing a 2D game, I tend to focus on the edge of the screen, whereas with a 3D game I tend to focus on what's happening right in front of my character.

I think "dumbing down" is really just a loaded term intended to get people upset. It's really just a different gameplay style, one focused more on action than on contemplation. It's possible to think of a purely turn-based game as just one big game of chess, with opportunities at each turn to take a break, think about the next move, etc. Some people are bored or irritated by that stuff and don't feel as immersed as if they are "in the game," running around with a sword or some such. I'd rather feel like a general playing these games than a soldier, which is probably why I prefer games like Company of Heroes to Call of Duty.