We have to take into account that Metroid has been in the darkness for quite some time. The last big entry (and probably the biggest one ever in terms of budget and scale) was Other M in 2010, and it barely sold over 1 million. After that we only got Federation Force, which bombed very very hard and a remake of one of the most obscure numbered Metroid entries on a console that was way past its prime. That one sold like 500.000 units I think.
Dread selling anything between 3 and 5.5 million lifetime is an absolute win. Not only will be the best selling game on the franchise, it will be remembered as one of the biggest comebacks in gaming history.
BTW, completely unrelated, there's something about Mercury Steam that I find quite curious. They have worked with the two franchises that give name to the Metroidvania genre. However, when they worked with Castlevania, they stripped off its exploration roots and turned it into a linear 3D action game. But when they had to work with Metroid, they were very careful about being faithful to the franchise's roots.