It seems like Rare is getting ready to move away from Sea of Thieves to some extent, possibly to focus more on the development of Everwild, which was announced in 2019. Fortunately, this does not necessarily mean we'll get less content for the pirate simulator in the future.
The developer has now confirmed that they will team up with the Liverpool based studio Lucid Games (Destruction Allstars) in developing the game, and the latter tweets:
"We're excited to announce that we are working with @RareLtd on @SeaOfThieves! It's been incredible supporting them on their thrilling pirate adventure & we can't wait to see where this legendary voyage will take us!"
Exactly how many Rare staffers that will still be involved in Sea of Thieves is currently unknown, but a qualified guess is that it will mostly be on a consult basis, as it sounds unlikely they would announce a collaboration like this if it was only about a handful of people. Hopefully we'll get more information about this fairly soon.