It's just a rough guess, I took into account missed purchases by Wii U MK gamers, but also positive factors like the almost sure fact that Ninty won't kill NS early as it did with Wii, and also that the hybrid concept, just like motion control in Wii, is attracting new users that previously didn't buy Ninty home consoles. Moreover, on a hybrid MK8 can offer gamers its full potential, as single player both at home and on the go, and as family, friends and party multiplayer, for new users is a lot of value.
Other negative factor: MK Wii reached 25M sold 3 months faster, durng Xmas 2010 period. Other positive factor: after that Xmas Wii started a quick decline, had much weaker Spring and Summer than NS and successive Xmas was still strong, but considerably weaker than the previous one, while it's very likely that Xmas 2020 will be very strong for NS. Another positive factor: Wii had a 80M user base when MK Wii reached 25M, NS is around 60M now and it was some millions less on 31st March, MK8D attach rate is currently much higher than MK Wii's one was both on Xmas 2010 and on 31st March 2011.

Surpassing MK Wii is pretty much guaranteed at this point. As Alby_da_Wolf said, MK Wii passed 25 million when Wii hit 80 million, while MK8D is passed 25 million at this point with less than 60 million Switches. Furthermore Switch sales from this point onward will be far strong than the Wii's sales were. MK8D should sell quite a few million more than MK Wii did.

There's no way this game sell less than 40 million.
I expect almost 50 million

Mario Kart 9........ but it will still sell 40mil lol

I went with 35-37.4m and I do believe Mario Kart 9 will be release on Switch, maybe for holiday 2021.