Mario Kart games have long legs, so I doubt Nintendo would put out one fairly late in the Switch's life which would not only slow sales of MK8DX, but probably also not sell as well on its own because a lot of people will just be like "another Mario Kart? But I already own Mario Kart Switch".
Even if they DID make one late in the life of the Switch, it would probably get a superior/enhanced port on the Switch Succ anyways, so why not just wait a little bit, let the game build hype for the new console, and give the game the full length of the new system's life to sell like crazy.