Can't help but wonder if there will be a MK9 in the next 5 years.
Definitely seems like the same situation as Smash.
Nintendo releasing DLC that's worthy a game+DLC of content 5 years after the game launched(8 years after the original launched) makes me lean towards no.
Drake being backwards compatible will make Mario Kart 8 Deluxe its Mario Kart too, as both people that currently own Switch and make the jump will have it, and newcomers will buy it.
I don't see a scenario where they go berserk like this releasing DLC 5 years after the game released just so they release a brand new game the next year after the DLC is done(like, DLC until late 2023 and a brand new game spring 2024 sounds insane to me, and I use to be quite optimistic on release dates - I was among the people thinking Zelda was 2021 lol).
At this point I think they can go the route of the GaaS games and releasing a big update alongside another paid expansion that makes the game another entry in all but name.
Dream case scenario to me:
After wave 6 releases holiday 2023, they announce a new booster pass that also includes new characters and more new tracks, maybe instead of a 6 wave pack they go with individual 2 cup packs like the Wii U DLC was handled.
The date the paid expansion is released also comes with a big free update that includes something major like a new game mode(story/mission/campaign please) - the way that the last free update + Happy Home Paradise was in ACNH.
I think a second season of DLC, each wave being cross over like the Wii U would be great - a wave with Samus, a Metroid course and the other tracks mixing old and new; then a wave of Kirby, F-Zero, etc.