Let's not bring up our past predictions. Remember, you're the one who said Wii U would do what, 40 million?
Anyhow, MK7 isn't selling 15 million. At most it'll sell 12.5 million. The 3DS is slowing down, and so are it's SW sales.
While 12.5 million isn't a ludicrously low estimate, it's a very low estimate for its maximum potential sales.
Of all the sales Mario Kart DS would have after week 100, weeks 100-150 accounted for 23%.
For Mario Kart Wii, weeks 100-150 accounted for 51%.
In order for Mario Kart 7 to fail to sell 12.5 million, weeks 100-150 would have to account for at least 60%.
Keep in mind that the Wii's drop after week 100-150 for MKWii was much larger than what you can expect the dropoff for the 3DS to be. If the 3DS were to have the same dropoff, it would sell 7 million over the next 12 months. That's an insane low end.
In other words, 12.5 million isn't completely impossible, it's certainly a potential low-end, but there's really no way you can claim it's the absolute high end.
I'm not even accounting for the digital sales of the title here. I have no knowledge of what they are, but the inclusion of them make 12.5 million an even more ridiculous high-end.