Let's be optimistic (and actually somewhat reasonable). If we go ahead and say 33% of all CURRENT Wii U owners purchase the game, which is fairly reasonable considering who likely owners of this system are, and we KNOW that a bunch of people who don't have a Wii U definitely want this game, it's highly likely to exceed 3-4 million units by the end of the year. Wii U hardware will see a big boost because hey, the game advertises the Wii U (assuming Nintendo can keep the information and marketing flowing).
Now, if what I think will happen actually happens, we could see Wii U pass 10 million, even hit 13 million units by the end of the year if steady releases continue to launch. My hunch tells me that MK8 was ready to hit by December, but back at E3 last year, the Big N planned to have their 2014 titles more consistently paced and so planned the launch of their marquee title to kick off the stream. We'll already see Bayonetta 2 hit Japan in June, which makes 2 big hits month after month there. We still have yet to get dates for X, SMTxFE, Yarn Yoshi, Hyrule Warriors, Smash, or Zelda (2015 likely on that last one). That's five 2014 titles on the horizon, with seven months left in the year. Easily prepared for a stream.