MK8D is like 16 million by now in 2 years so I think Nintendo will wait until it reaches MKWii's 30 million before considering a (useless) sequel
As of March 31, 2019...
16.69 mil for Mario Kart 8 Deluxe
8.44 mil for Mario Kart 8
That's 25.13 million units.
Both versions are the best-selling title on their respective console. Nintendo really should release Mario Kart 9 by 2022 or sooner. It would sell for sure, especially if it comes out when there aren't many other big games.
So many people think you can't add much more to the series. Give me some new courses (Super Mario Odyssey, hello), new characters, new items, Double Dash mechanic, missions, and I'm all set. You do all those things and market the heck out of the game and it sells 10 million at least.
Then 1-3 years into the Switch's successor release Mario Kart 10.
Lifetime Sales Predictions
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 110 million (was 73, then 96 million)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima