I know I've said this before, but I'll say it again because it seems to be a very under-represented view: I can "understand" the 3D games just fine, and I've beaten them, but I still like the 2D games better for various reasons.
At least, I have up to now. The 2D Mario series has lately been in the process of de-volving into a bunch of dull, unimaginative rehashes. It seems like the people at Nintendo are either seeing people's fondness for the traditional structure and mis-interpreting it as a need to hardly include anything new in order to avoid spoiling the formula, or else they're purposefully trying to make the games appear lackluster compared to the new 3D ones. I don't know much farther they can go down this path without the 2D series suffering significant sales decline (I believe NSMB2 is already underwhelming a bit).
As the developers at Nintendo have long held the idea that their games don't fail because of any problems inherent in them, though, but merely because people don't properly "understand the value", I wouldn't be suprised if they let the series decline and chalk it up to people not understanding the games properly. Sheesh, Miyamoto actually took some time in a fairly recent Iwata Asks
interview to explain that the only reason for Virtual Boy's failure was bad marketing.
I don't know whether to laugh or cry at that.