Yeah, Luigi's Mansion sold 13+ million to the expectation of no one. It also stars Luigi. If that's the parallel, the flagship launch title of a new system is not when Nintendo gambles on a longstanding IP jumping up one, two tiers. If anything, Nintendo will get more calculated for a system launch.
It's not like Metroid is invariably fixed at 3 mil, MP4 will probably exceed that at some point, but if anyone is talking out of their ass, it's someone who is casting Metroid in the headlining position Mario or Zelda usually take and acting like it's crazy that people would find that replacement questionable.
Metroid isn't some obscure series by any means, but it's a far cry from Nintendo's top tier commercially and demographically. Especially in Japan, which Nintendo cares about. Not knowing much about the game, not having got one in a while, Retro being talented; the reasons people aren't expecting it to be a juggernaut aren't arbitrary, the reasons people are expecting it to be a juggernaut are arbitrary.