I dunno... even if it manages outstanding Holiday sales of 3 mil globally, it would still have to sell more units in its first full year than the 360 or PS3 sold in their peak years. It COULD happen, as it did with Wii, but it would be very impressive.
Mind you, no matter how well it sells, it will be "a disappointment" and Nintendo will be doomed. Suppose it DOES sell 18 mil -- as you pointed out, Wii sold 1.5 mil more than that. So Wii U will be "not as big a success as Wii" and not very impressive compared to its predecessor. If Wii U sells 20 mil in that time period, that's only about 500k more than the Wii sold in the same period, which won't "represent enough growth from last generation" and will be viewed as a failure. If it sells 25 mil, people won't be impressed because the DS did better than that 3 years in a row. And so on.
