From about 2011 onward, I saw it as the normie Mario Kart. While I played Double Dash, DS, 7, 8, etc. so many people think of Mario Kart Wii. I don't see MK8D outselling it, but MK8D is doing insanely well for a port. I wouldn't have predicted it would be the best-selling game on Switch.
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).