Yeah, makes sense. Let's kill the only franchises that sell well xD
Smash has a lot of room for new ideas and mario kart even more. Adding more characters which aren't from the mario series is not a bad thing. Imagine a splatoon themed track, a kirby one, pikmin 4, metroid, even xenoblade! I'd love a course in gaur plains with monsters getting into your way. And that godly music...
And with 2d mario they can still do so much more. For me, Mario as a franchise is almost infinite.
They will be fine! :)