Metroid Series gets amazing reviews, yet the series usually has a hard time selling compared to other series of lesser critical acclaim.
Metroid is the Series that started off the Metroidvania genre. I know the series has gained in popularity over the years/decades due to Smash Bros, Amiibos, online fan flare, & high scores on metacritic's (and sites alike) database/s. But the games generally don't sell as much as they should... all though I may be out-dated and wrong in 2023 as Metroid Dread has sold more than any other Metroid game and I predict Metroid Prime 4 will be the best selling Metroid game up to date when that comes out.
I mean, they might not be the best-selling games ever, but it's a bit of a stretch to say the Metroid series is the "unpopular apex" of the Metroidvania genre.