People think 2D Mario is a fad again. The same thing happened back when the NSMBW was announced. I think some people here weren't into sales at the time. So they don't really understand just how big the Wii and DS 2D Mario games were.
They look at the 3DS and Wii U entries and think that the games are a fad. However, they went down because of the downturn of quality not from the lack of demand.