Wii software sales sold over 1 million for FY2019, topping the Wii U's 850K for the same time period.

And that's really what it was. Mario Kart Wii was when they got the foundation for the series right. Calling it the normal Mario Kart, or the first modern Mario Kart would also be a fair description.

Mario Kart 8 Deluxe is also perhaps the main piece of software driving Switch sales right now. I work in an office, and we play Mario Kart 8 religiously at lunch no less than twice a week (and five times a week when the game was new) since it launched, others see us playing and want in, so they get a Switch. While this is my own anecdote, I can't imagine that this story is not a common one; perhaps a little different depending on the situation, like University campus hangouts or grade school recess waiting places/cafeterias (whatever kids do these days).

I still find it mind-blowing that new copies of this game are selling. With over 37 million units in the wild surely there are enough second hand copies floating around to meet any remaining demand? And who is buying them? Nintendo stopped manufacturing the Wii in 2013.