It's possible that Mario Kart 8 Deluxe may not be as "Deluxe" as we were led to believe, as new content is coming to the game in the future that may include paid DLC.
Shinya Takahashi revealed in an interview that was uploaded to Nintendo of America's Twitter page that "further updates" will be coming to Mario Kart 8 Deluxe in the future, though he didn't specify what form those updates would take or when exactly they would be coming.
There are several forms in which these new updates could take, but the most likely will be paid DLC. If Nintendo had plans for free updates for the game, then we would have heard about them a long time ago, as they have used that tactic to promote some of their games, as was the case with Arms and Splatoon 2. The promise of free content updates is a big allure to people who may have been on the fence about buying the title again if they had already purchased Mario Kart 8, so it would be off if they started to release some after the game has been out for a year.
It's also possible that the updates could be linked to amiibo support in some way, as Nintendo has promised to continue supporting amiibo in the future. A series of Mario Kart amiibo would probably be very popular and it's odd that Nintendo hasn't already tried to release some.
Mario Kart 8 Deluxe recently received a new update that added a control scheme that could be used with the Toy-Con Motorbike, so it's possible that the updates may be linked to an upcoming Nintendo Labo set. The current Labo set that is compatible with Mario Kart 8 Deluxe is intended for riding a bike, so it's possible that we may see a wheel & peddles kind of arrangement in the future.
Mario Kart 8 was the best-selling game on the Wii U and Mario Kart 8 Deluxe has already managed to outsell it. Mario Kart 8 Deluxe is the second best-selling game on the Nintendo Switch (behind Super Mario Odyssey), so it wouldn't be too surprising if Nintendo wanted to release DLC packs for the game.
The future of the Mario Kart series in this current generation of consoles may be tied to paid updates, as Nintendo has yet to release more than one Mario Kart game on the same system. Mario Kart 8 was likely an easy choice for a Switch port, due to the amount of DLC that was available for the game which could be bundled into a new package, but it would diminish the idea of Mario Kart 8 Deluxe being the total package if players were expected to pay for content in the future.