Having only just read through this, I'll start by referring back to the argument about the Wii, and whether its success was down to the hardware selling software or vice versa - both sides of the argument are wrong, IMO. The Wii and all its sports related games were cleverly marketed together as a package - the hardware and software sold each other. That was what underpinned the Wii's success and grabbed the imagination of the public over the first couple of years after launch.
By the time Mario Kart Wii was released, the install base was already 20 million. The game's lifetime sales went on to 34 million on a console that topped 100 million. Mario Kart, because it's a racer, has a wider appeal than the side-scrolling Super Mario Bros series. New Super Mario Bros Wii only managed 27 million in comparison.
In contrast, the Wii U suffered an identity crisis, and what little marketing there was, came across as confusing. The damage has been done, and rather than save the Wii U, Mario Kart 8 is now all about damage limitation. However, I think that the Wii U will benefit more from MK8 than some might believe, simply because of the huge exposure and appeal Mario Kart Wii had. At this moment in time, Mario Kart 8 is the biggest reason for buying a Wii U, and bundling the game in with the console is a smart move.
Just today on Amazon UK I have seen the MK8 hardware bundle rise 8 places through the pre-order charts to number 43 (at the time of this post) and the old white Wii wheel jumping up 200 places. And that's in a country where there has been little to no interest in the Wii U.
It would be foolish to think that every Wii U owner has a Wii U because they are a Nintendo fan, and that only Nintendo fans will by MK8. I have seen enough posts on these forums to know that people do own more than one console, and that people do buy the Wii U as a second console for the Nintendo exclusives.
I don't like to predict, but I feel confident that even after the inevitable bump after release, Wii U sales will eventually settle at few thousand a week more globally than they are at the moment, especially if Nintendo keep to their promise of continual advertising through to the end of the year. Although the Wii U will never be the phenomenon the Wii was, and may not even live up to any of its other predecessors, the impact of MK8's release could well be enough to positively impact the sales of existing and subsequent games on the system.
In short, I don't think MK8 will 'save' the Wii U on its own, but it sure has the potential to get the ball rolling.