All I'll say is that I'm baffled as to why people would be unhappy with a new 2D Mario game. NSMBU looked gorgeous, with ghost levels that looked like a Van Gogh brushwork level in the background. Given how fun NSMB Wii was, especially the last level and endgame, I don't know how people could be disappointed in it.
OK, sure, we had two very good 3D Mario games with Galaxy1/2. No arguments there. But like the argument over which Zelda gets on 3DS (Majora, LTTP or a whole new one), my answer to both propositions is a wholehearted yes to any and all of them, so long as quality is maintained.
And there is no reason to assume it is dropping off, given the recent series entries.