My take is that there are basically three directions they could have gone with after NSMBU:
1. Make another New Super Mario Bros. game.
2. Make a new 2D Mario subseries that isn't New Super Mario Bros, but sort of uses NSMB as a base. Expand it a lot from there, make things stylistically different to a degree, add in unique gameplay variations, it's very much not NSMB, but it's still built from there.
3. Make a 2D Mario game that essentially acts as though it was made in an alternate universe where NSMB never existed. Go crazy with the style, the presentation, the structure, hell even the physics (maybe make it more like the 2D sections in Galaxy 2 or like if SM64 was in 2D).
To me, this is clearly 2. 3 is a game of my dreams, but 1 or 2 is realistically what we were going to get, and I'm happy with it.