It's certainly delayed the development of MK9. I believe this is one of the reasons we got MK8 Deluxe
We saw this actually happening with MK8D and Splatoon 2. Now Pokken joined and the main reason why there's no Mario Maker for Switch yet is that it's already on 3DS. Nintendo explicitly stated they're considering more Wii U ports for Switch too.
Thanks to the Wii U library, which still has room to grow on Switch, Nintendo has their hands free to do more new IPs. Which they've always done but they were spread among two systems and the established IPs requiring new installments limited how much other projects they could work on.
New IPs are more interesting, both for players and devs. I don't know what new gameplay mechanics you even expect for yet another kart racing game. A new F-Zero would be more interesting than MK9 (only problem being, it wouldn't sell). In case you're fine with more racing tracks for MK, there's always DLC for that.