This is an interesting question - and one that we actually can make a pretty solid analysis about.
Metroid Dread is Metroid 5, there are four previous games in the series to look at:
- Metroid (NES) → 2.73M
- Metroid II: Return of Samus (GB) → 1.76M
- Super Metroid (SNES) → 1.42M
- Metroid Fusion (GBA) → 1.68M
So, the first thing to notice is, that the first game sold about 1 million more than the others. Excluding the first, Metroid actually lands in a pretty narrow sales corridor of 1.4M-1.8M. This points to a small, but enthusiastic fanbase. So the basic assumption is, the new game will move in the same corridor. I will discuss a bit later, inhowfar the new entry may break from this window, but it should be a good base to start the prediction off.
But first there is need to explain the outlier. If there are no special factors that boosted the first game so much more, maybe the fanbase actually is more volatile. But I think these special factors exist. Metroid is a game, that together with another game named a genre. This tells us two things: first that the style of gameplay is interesting enough to be separated into an own genre. And second, that probably no major game existed before Metroid and Castlevania to follow that style of gameplay. Which is reason why the first game is an outlier. As Metroid dropped it was a fairly new concept with groundbreaking mechanics. Everyone needed to play it. At the time the sequels arrived, quite some other games had arrived, that were based on similar mechanics and some people that bought the first Metroid may have decided to stick to one or multiple of the alternatives (Megaman, Kid Icarus, a lot of Castlevanias and so on).
So, there is reason to believe, that 1.4M and 1.8M is the corridor the sales of the series will reside. Let's look at possible reasons why Dread may break out of this corridor.
For one is the Switch factor. But we can actually be more precise (which also explains why some games profit more from the Switch factor than others). The main reason games get more attractive on Switch, is that it supports both handheld and TV play. This means that both fans of Metroid as a handheld game (like Metroid II and Fusion) as fans of the TV based games (like the first and third one) are content to get the Switch game. And some may even prefer Switch, as they want to play it in both styles.
The second reason is the time. A long hiatus can be damaging for games sales or helping. Damage mostly comes from less exposure. If the game falls into obscurity, a sudden new entry may not be well received. Is Metroid fallen into obscurity? I doubt that. And there is the other side of the coin. If games of one series release too frequently, some customers may feel burned out on them. Happened to me for Assassins Creed and Pokemon. But Metroid 5 follows 19 years after Fusion. So people that are interested in Metroid hunger for a new game.
The third reason is quality. People dismiss this fast, but quality actually positively influences sales. As people tell about the game other might be interested. But if they tell about bad stuff, others might pass. To be fair, so far we only have a trailer, but at least what was shown there looked like it had a good level of polish. If that holds true for the game as whole, the game may be well received.
Fourth reason is the growing markets. Since the days of GBA Nintendo expanded their presence in europe and china. Finding new customers in the regions might help to sell a few copies of Dread extra.
With all that I think Metroid Dread may break the sales corridor in direction of higher sales. I am not sure how much, but if I have to say a number, I say 2.6M, just short of the fist game.
