Wasn't the same true for the DS and 3DS Pokémon games vs. Animal Crossing Wild World and New Leaf? It just doesn't matter how crazy Animal Crossing's legs may be, Pokémon always sells insane numbers and it'll sell even more insane numbers on the Switch.
Also, X/Y sold 4 million copies after the release of OR/AS, so no, it's not like the game's sales just stop completely after the next release.
I just think Animal Crossing is going to be bigger than it has ever been
But it's also possible, maybe close to likely, that Pokemon can be bigger than it has ever been, since Gen 2, possibly, as its Gen 8 on the Switch? Pokemon Let's Go was able to sell 10 million in 2 months as sort of a B-tier type of game.