Wii U sales trend may be just as expected if we look at Nintendo's home console sales.

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AnthonyW86 said:
Indeed this trend is clear to see. More importantly if Wii-U sales really end up at around 15 million, keeping some of the biggest gaming franchises like Mario exclusive to such a small install base is nothing short of insanity.

Mario's on 3DS too, and that's already sold over 44 million.