TOTK will have already hit 20m, it was already over 18m at the end of June.
Ordinarily I'd say Mario has this in the bag as 2D Mario is a juggernaught, and Wonder will be releasing to ten years of pen up demand. It's going to blow passed 30 million for sure.
But open air Zelda is a titan in its own right; Link won't go down easily.
Let them fight.