Dread did 2.8 million in 3 months. It is basically the best selling Metroid game in the series in only 3 months.
People think that Metroid needs to sell 10 million+ to become a hit. No it doesn't. It takes time to reach that, and a title can be a massive sucess(relative to what it normally do) without doing Mario or Zelda numbers.
Like I said, Dread is the best selling Metroid game already, and considering the legs of Switch games, it will probably do more than 5 millions lifetime.
So while Metroid 8, far into the future, mught do 10 million+, for now Dread IS indeed the breakout hit the series desperatly needed.