I don't really see the Wii U turning around. I don't know what the western sales for Mario Kart are like, but in Japan, it wasn't a smash hit like I thought it would be. Sales dropped hard and fast. Though I'm sure the legs will be quite long, given the install base of the Wii U, how long those legs are might be a matter of perspective.
If you look at just percentages of Nintendo console owners to MK owners, we are more or less at that ceiling already. That is- most people who are planning on getting Mario Kart already have.
Furthermore, only a handful of major games for the last 6 months of the year, and only one of them with real sales potential. As much as I'm looking forward to Bayonetta 2(+1!), it's not a system seller. Or a self-seller really. A bizarre game with a very particular target audience.
I feel like the writing is on the wall, and Nintendo knows it. Virtually zero third party support, and the major first party releases are few and far between. I suspect that at e3 2015 we will see the successor to the 3DS, and by extension, the successor to the Wii U (assuming that the hybrid console rumor has any validity), with an eventual release in 2016. I feel that a four year lifespan is acceptable for a console that has performed as the Wii U has.
We already have Mario, DKC, Mario Kart, Smash, a big JRPG, Yoshi, Kirby, Zelda, etc targeting late 2014/2015. All of the major IPs have launched or will launch soon, and 2016 and beyond is going to be pretty desolate.
The wisest thing to do right now would be to take a feather from Sony's cap, along with a slice of humble pie, and start asking developers and consumers what they want. Because it obviously wasn't a gamepad. Take those thoughts and ideas, and channel them into the R&D department and get something together for the future. Another Wii U size failure will probably kill Nintendo's hardware business.