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.
Yeah. I remember when they said Breath of the Wild was already making a profit, back when it sold like 2 million. If that was true, I can't imagine Metroid Dread needs much more than 500k to break even. Even if it needs more than that, I believe Samus Returns sold like 600k or so? Pretty reasonable to expect Dread to sell twice as much at the very least. More likely it sells at least 2 million.
...And agreed on the technical/creative difficulties thing. It's pretty clear the 2D series died because Sakamoto only wanted to continue it with Dread, so much so that he's still doing this project over 15 years later. Meaning if he could never make Dread the way he wanted to, then the main series just wouldn't go on. When he says MercurySteam convinced him due to their work on Samus Returns, I also kinda wonder if Other M was also like a test round for Team Ninja, which obviously didn't work out.
The Prime games are in a bit more of a difficult position with everything that went down at Retro, but it makes sense that they wouldn't want to make anymore after Prime 3. It's pretty clear in that game that they really wanted to give that storyline a proper ending, and have it be a proper trilogy. I think we're only getting a Prime 4 due to fan demand really, which makes me concerned it won't feel like a genuine work of art - a worry I don't have with Dread.
Either way the Prime games are where the sales thing becomes more of an issue, because those have had insane production values for the kind of sales they brought in. When your AAA level games are only selling nearly as much as the 2D pixel-art ones, well, that's kinda rough. Heck, just look at the development for Prime 4. Since its initial announcement, we're yet to see any footage of it - in that time, two whole 2D Metroid games released. Prime games are obviously a lot more expensive to make, so it makes sense Nintendo would be more careful with it.