I doubt that Nintendo will release a Mario Kart 9 for the Nintendo Switch with Mario Kart 8 selling as well.
But I'm kind of sad about it, I have Mario Kart 8 since 2014 and I saw no reason to have it again for the Switch (to be honest, I got tired of the game on Wii U in 2016, I even went back to playing Mario Kart Wii because I prefer his track selection.), battlemode is cool, but not enough for me to buy the same game again.
It's kind of boring to see Mario Kart Tour on Mobile receiving so much new content and nothing for the Switch, I really wanted to see a new Mario Kart with the same treatment as Smash Bros Ultimate, making the most of the hardware, having content from the latest games from Mario like Super Mario Odyssey and Origami King, maybe even Labo VR support, but unfortunately I think that in the end, an incredible console like Switch will only have a simple port as the main Mario Kart game on the console.