For all we know, some franchise keep increasing his popularity, others decline at time passes. So in order to think about the possibility of 10+ sales, the sales of the Wii U need to improve dramatically to accomplish that, and not based the 15+ million sales based on the Wii version only.
I predicted 10 million sales based on the current and expected future Wii U sales, i could be wrong, but it's still a prediction and there's nothing wrong based the prediction on every historical data and not the lastest one.
There is if you're ignoring the changes over time in the franchise. The franchise has had tremendous exposure over the last 7 or so years. 30 million people played the franchise on the Wii, and 20 million on the DS which definitely changes the outlook of the franchise. The most recent entry in the franchise, Mario Kart 7, is actually outpacing Mario Kart DS, which does not seem to indicate a franchise that is declining.
Obviously Market share is going to play a big factor, but Mario Kart 7 should comfortably hit 10 million on an install base of about 40-45 million, but Mario Kart has, even in the Gamecube days, performed better on home consoles due to the draw of multiplayer, so 8 may be able to surpass 10 million with 35 million consoles or so. Of course, I think the Wii U will sell much more than that, and you seem not to, but that's an argument that's been done to death, so I'll just post my predictions and we'll see who's right in a few years or so.
Super Mario 3D Land- 8m
DKC Tropical Freeze-4m
Mario Kart- 18m
Smash Bros for U- 9m
X - 2.2m
Killzone Shadowfall- 3m
Infamous Second Son-2.4m
Drive Club- 0.4 m not including digital
Dead Rising- 1.8 m
Titanfall is the hardest to really guess on. Honestly, I could see anything from under 3 million to 8 million happening with that one.