I think a large part of how Metroid turned out was Nintendo overseeing development and giving pointers on how to make everything better. It wouldn't be the first time this happened either, we have seen it countless of times with different studios. So a new Castlevania game by MercurySteam may not be as good as Metroid Dread. However, any game they make with Nintendo will most likely be an excellent one and, hopefully, they make the next Metroid game. They can probably work in more than one game at a time too, but I am not sure their other projects have the quality of Dread, maybe it's also because Nintendo is very lenient with release dates, so if a game needs more polish they are willing to delay it so it can be as good as it can be, other publishers aren't so kind (I am looking at you Sega).