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
PS4: 130 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 96 million (was 73 million)
3DS: 77 mil (was 73 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima