The Wii U needs to average about 97k sales / week to sell 5 million this year. Since demand curves are not linear, we have no idea how a $50, $100, $150 price cut will affect sales. Last year it averaged 70k/week and currently it is up YoY by about 12-15%. If it consistently remains up by 12% on average then it will be at the 80k /week mark. That means 4.160 mil sales. However, with a price cut, and the release of unique games which can bring in new players: Zelda U, Xenoblade X, Splatoon, etc the Wii U can indeed reach that 97k average we are hoping for, if it has a good holiday induced by a price cut + a critical mass of game library. Maybe even a redesign can help. I expect the Wii U to sell somewhere between 4.5 mil and 5.5 mil this year, and I also expect this year to be its peak year. Smash Bros and Mario Kart are still helping it, as they are games with legs, and the library is diversifying even more. We all saw with the PS3 what happens when time is given for its library to accumulate, and the price drops.
5 mil is not out of question, to be honest.