I think it can sell over 10 millions this year.
I think Mario Kart, Smash Bros and Zelda (probably released in 2015) have the power to pull the Wii U to a Gamecube level of sales, each of those titles will be bought by a few million dedicated fans, and many others will buy a Wii U not for 1 title, but because it would have accumulated too many "must haves".
This would be a scenario of Nintendo depending on hardcore gamers to survive, so, the games would need to be good, the Zelda would have to be better than Skyward Sword.
For most gamers Wii U will only have a decent enough library to deserve buying it in about a year. PS4 and Xbone may have no games now, but everybody knows they will get all the third party support, Unlike them Nintendo consoles have to prove they will have games.
And some surprises may happen, if 10% of the housewives who bought Wii Fit twice for Wii buy a Wii U, that would be over 2 million Wii U's sold no one is counting on. It may or it may not happen, no one knows.
Donkey Kong Country returns sold 6 million on Wii and 1 million on 3DS, perhaps an important ammount of those customers will want a Wii U for Tropical Freeze.
I think the next 2 years could be quite good for the Wii U, but after that it will go back to selling less than 5 million per year, because Nintendo would have spent most of its ammo, Mario platformers, Mario Kart, Zelda and Smash Bros. They would only have the smaller franchises like Metroid remaining.
A new Mario platformer would be put to better use on a new console methinks.
3DS is already 3 years old, I see Nintendo being done with the Wii U at the same time they stop supporting the 3DS.







