I'm not sure one title can save Wii U, but if one has a chance to at least stem the bleed a little bit, it certainly is Mario Kart. Maybe this can keep the Wii U floating until Smash Bros arrives at Christmas.
Honestly, the Wii U is starting to look like a bit of a better purchase these days. Mario Kart 8 is getting rave reviews, I want Wind Waker HD, Smash Bros looks good, and the (brief) Zelda footage we saw at E3 really gave me hope that the series is going to move in the right direction.
The necessary critical mass still isn't nearly there yet, but there's at least some positive movement here.