Sony will beat Nintendo just because their formula for a console is far more balanced than Nintendo's. Nintendo relies on a narrow range of franchises to sell the entire platform tied to mediocre third party support and out of step hardware. The Wiimote's insane success masked a lot of the problems Nintendo had with the console market, but now they're coming back to roost.
I just think more Mario doesn't address the main actual problem the Wii U has, which is that this crowd:
Ain't coming back. The 20-something hipster crowd before they got their smartphones was a huge driver of the Wii, and the Wii was the "hip" fun thing to have for a few years with Wii Sports for house parties and get togethers.
The Wii Fit soccer mom crowd ain't coming back either, diet/excercise fads are called fads for a reason. In all honesty a light jog around one's neighborhood for 10-15 minutes a day is probably a better workout than standing on a balance board.
I think as well the last few years of the Wii were handled incredibly poorly and a lot of people who were enthused by the early days of the system left it gathering dust the last few years.
These are Nintendo's main problems, and no amount of Mario is going to fix it IMO. Mario Kart may be tempting to some but without the Wii Sports/Fit craze I think a lot of casuals are going to pass. They played MK already on their Wii and the new one is basically the similar game just now with prettier graphics.
This is the audience Nintendo is probably more likely speaking to, but even this guy is a bit of a stubborn sell for Nintendo because this guy's accustomed to not paying a lot for Nintendo hardware and knows they cut the 3DS price quickly so he is standing pat. But Nintendo will eventually win this crowd over, just a matter of time.
Nintendo simply just has niche appeal in the console ranks when stripped of the casual gaming safety blanket. Sony/MS are just more mainstream, have more kinds of games in different genres, better marketing, far better developer support. One developer, no matter how good they are can't make up for the entire third party community. It's a formula that simply can't compete in the long run, or even in the short run IMO unless it's tied to some kind of Wiimote/Wii Sports phenomenon, which the Wii U doesn't have.