I don't think sales are the Metroid series' problem. As long as the games sell in the millions Nintendo seems content to make them.
It seems to me that the lack of development of new Metroid games had more to do with internal workplace shifts which hindered the ability of Nintendo to produce good Metroid games.
Retro Studios wanted something new after Prime 3, and a lot of the old staff had left anyway. Every new developer Nintendo worked with ended up shaping the series into something fans didn't like. The previous attempts at developing Metroid Dread reached technical or creative limitations.
That Nintendo decided to develop Dread after Samus Returns' sales signifies to me that they are happy with the games in so much as they break even profit-wise. Whether the game is well recieved seems to be more important to them.
This makes sense too. Nintendo understands that their core base needs to be excited for other people to be excited about their systems and games in the long-term. Metroid might not be a huge seller in itself, but it has a strong fanbase that almost 1:1 overlaps with Nintendo's and this helps ancillary sales of other games in that it brings word of mouth/media coverage to the system in general.
I still think Dread will probably sell over 2 million, but if it sold only something like 1 million I don't see Nintendo having a huge problem with that.
The game's development costs probably are mid-tier/"AA" level.
The funny thing is I 100% agree with this when it comes to Metroid, I'm not trying to sound crazy when I talk about sales mattering. There's something about the series that I think it's reasonable to say makes it more important than say F-Zero or Golden Sun to Nintendo, and I think it's that it probably has a higher unit-pushing ratio then say F-Zero (that is to say, probably more people would skip out on Switches if no Metroid than is the case with F-Zero, even if both series are relatively niche) and also that Metroid is just one of their most acclaimed series ever. It's sort of like how for a long time Sony felt it necessary to platform Team Ico games.
However, and maybe this is a poor assumption on my part, I do sort of think that might change with theses games. If these games didn't sell as well as some of the peaks of prior Metroid games (for example, Prime 4 not selling as well as Prime/Metroid 1, or Dread not selling as well as Fusion/Metroid II (I don't think it selling worse than 1 would be their breaking point), that would indicate a surprisingly low amount of interest for a series that for the first time in forever is having all the stars align.
Honestly, the likelihood of that even happening (sales being that low) is pretty small to begin with. And honestly, I wouldn't be surprised if they just continued treating Metroid much like how Sony used to treat Team ICO, and in that sense I'm not putting all my eggs in this one basket. I just wouldn't be surprised if it changes for these games and, given super mediocre performance, it goes on hiatus again. Not out of some dismissal of the franchise, just out of it not being a big priority.
TLDR: I agree with you and have always thought about Metroid that way, I just think it's possible that might change for these games. And moreso, my point is that if they got super mediocre sales, I wouldn't blame Nintendo for putting the series on another long hold. Not that I personally think that will happen.