It's very possible, but not a given. 15 million lifetime is the bare minimum, basically. It will probably already sell 15 million by the end of 2020 or at least by the end of Q1 2021.
I'd say 20 million is pretty much a given. If it somehow sells 30 million or more, I will be very surprised. Mario Kart 8 Deluxe, Super Smash Bros. Ultimate, and Pokemon Sword/Shield are the only Switch games I can see selling 30 million units or more. I'm sure Super Mario Odyssey and The Legend of Zelda: Breath of the Wild will hit at least 25 million.
Lifetime Sales Predictions
Switch: 144 million (was 73, then 96, then 113 million, then 125 million)
PS5: 105 million Xbox Series S/X: 60 million
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima