If they aren't planning on giving us a new Mario Kart it would make sense to release more DLC. The fact they seem to have no intention of doing so, seems to me like they do have another Mario Kart in the works that they will likely release towards the end of the Switch's life span, but I don't know. They already have the engine from MK8D, they could just reuse it and add new stages, but then again, that wouldn't be much different than just making DLC, it'd just allow them to charge more money for it. Which is a win for them I suppose.
On the other hand making a brand new engine to release MK9, would that really bring any advantages over reusing the old one? Doubt we would see much improvement in the graphics, the Switch isn't that much more powerful than the Wii U.
So I think it's likely we will see something at least, whether it's a brand new game with a new engine, new levels with a rehashed engine, or just more DLC. Any of those would be fine to me. But you'd think if they had plans to release more DLC we would have heard something about it by now.