It would have 20 million or more if it had a pokemon RPG in it. Nintendo is crazy for not doing home pokemon RPGs...
It makes sense as Pokémon is in its essence a very social game, and it seems like people still view it that way (e.g. Pokémon Go's success).
It's a good thing the Switch can finally fulfill the dream of the home console Pokémon game without straying from that essence.