At the rate the Wii U is selling I just do see it happending.
Right now actual sales of the Wii U are probably around 9.2million units or so world wide.
I can see the Wii U selling as little as 15million units before its replaced by Nintendos next (NX) consol.
I just dont see MK haveing that high a attach rate.
I can see MK8 hitting around 7-8 million units, if it can reach/keep 50%+ attach rate until next gen starts.