I'm not going to quote anyone in particular for having brought up the fact that Metroid isn't a "big gun."
But I will say that sometimes sales aren't the only way we can judge a game or a franchise's significance to a console's momentum and fan perception. While Animal Crossing will certainly sell more units and even has the potential to be a system seller, the simple announcement of Metroid Prime 4 arguably "won" E3 for Nintendo last year. In terms of hype levels it definitely raised the roof.
So, yeah, Metroid can't sell 4 million copies, but that doesn't mean those 4 million users don't become very, very vocal supporters of the console in a way that maybe an Animal Crossing owner wouldn't evangelize in quite the same way.