One thing I will point out, is that Nintendo always used 2D Mario to show off new pieces of technology - but the production values of the game itself were never elaborate. Super Mario World, for example, has very simplistic graphics and doesn't even use a large SNES cart. In terms of production values, graphical wow, it's conservative. It just showed off a few features like mode 7.
Only Yoshi's Island pushed for lavish, elaborate visuals in a 2D Japanese made Mario game.
In terms of technology, NSMB U is actually not unimpressive; it uses quite a few 3D background elements, some really huge 3D characters in places, and some unexpectedly artistic backgrounds with visual effects not really seen before. (Stylistically.) It's about inline with Super Mario World as scaled for the era it exists in.
Of course, the mandate of the NSMB series is "keep it simple, stupid", and to go for visual clarity for a wide audience. Now that the series is done for the foreseeable future, 2D platformers starring Mario might go in a different direction.
I actually find it intriguing that Good Feel is *not* making a sprite based sequel to Yoshi's Island or some such. With the way Nintendo thinks, that leaves artistically styled 2D platformers with an unfilled slot on Wii U, opening the possibility for such a game to be made in the next few years.
