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

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AnthonyW86 said:

You can't hook you're 3DS up to the big screen with 2-4 controllers and have a blast with friends and family. That's what makes games like Mario Kart and Smash Bros. so popular. But with such a small install base the amount of copies you can sell will be limited.

Mario Kart 7 on 3DS is still sitting pretty at 9.4m to date.