It all depends when Mario Kart 9 Comes Out. Let's say if it never releases on Switch, I believe MK8DX alone will outsell Mario Kart Wii Globally.
MK8 Deluxe is already outpacing the MKWii in sales within the same time period. And the Switch's install base has much more dedicated gamers than what the Wii had, so there are more potential buyers for MK8DX for the Switch, which is why I believe MK8DX will outsell MKWii.
But we have to see when MK9 comes out, if it gets released 2020 or 2021, I dont think it will outsell MKWii. But I could see Nintendo sitting on MK8DX for a while by how amazing the game is selling. I'm predicting towards the end of the Switch's life we'll see MK9 around 2023