At the earliest, a Switch successor is 2022 (given a 5 year life span at minimum), in which case we'd still go eight years without a new one.The difference is once you buy the DLC, you get all the content on the same game and can enjoy it with the full userbase. If you have to buy another game, you won't be able to make use of the old game's content in the switch (sorry for the pun) with the new userbase.
That could mean extended support with DLC effectively making MK a GAaS for the diration of the Switch's life, or it could mean they would still support the Switch for a while after the successor is released, much like with the 3DS, and the new game would land on the successor instead without waiting such a long time
I don't know, that part seems a bit unlikely to me. Plus ongoing GaaS is something Nintendo has shown a total lack of interest in, their support for a game caps at 18 months after its launch. I don't necessarily see that changing by much, maybe it becomes two years.
But we're a year out from MK8D's release, and so far, no peep of any additional DLC.