I'd love to get Mario Kart 9 on Switch, and it would be a big holiday title. But Nintendo probably wants to wait until 2021 or 2022 to release Mario Kart 9 if it comes out on Switch. Mario Kart 8 Deluxe is the best-selling game on Switch. While it likely will continue to sell very well, I don't see it being able to manage 6 million plus per year for the Switch's lifespan. I don't see Mario Kart 8 Deluxe hitting Mario Kart Wii's 37.24 million units, but combined sales with the Wii U version will likely match or surpass that feat.
MK8 deluxe alone will beat MK Wii.