Or you could look at MK sales as a proportion of install base.
MK Wii ~34%
MK DS ~ 15%
MK 7 ~22% (will probably fall over time but right now it's still selling at about 25%)
MK Double Dash ~31%
So let's be generous and say long term MK 8 will sell to 38% of Wii U owners. To outsell the current MK7 sales Wii U needs an install base of 25 million. And that is if MK 7 stops selling right now. Even if attach rate is 40% it still means Wii U needs to have an install base of nearly 24 million just to reach MK 7s current sales.
Seems rather unlikely given MK 7 will probably keep selling several thousand per week for as long as 3DS is selling. So MK 7 will probably climb towards 11 million over time.
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