Hmm, yeah I can see what you mean. Though I really liked the latest addition to the Tall Tales. The platforming and story was quite good. A little short maybe (2 hours or so?).
I'd love to see more interesting enemies and locations for sure. But for me and my friends (I think I play 2 times per month or something) it still gives us enough to do. We are playing through all the Tall Tales, doing some fortresses and sometimes PvP a little.
This is not true. My estimates are they sold 4 - 5 million copies at most. Last year, with 8 million people who've played, around 50% of them came from sales (
https://www.gamesindustry.biz/articles/2019-04-10-the-second-launch-of-sea-of-thieves /
https://www.gamesindustry.biz/articles/2019-06-13-sea-of-thieves-reaches-8-4m-lifetime-players). 10 million people have played Sea of Thieves so far. But Game Pass has become more popular. So 4 to 5 million sales would be a good estimate.
Meanwhile GoldenEye 007 is Rare's best selling game with 8 million copies sold. Sea of Thieves is somewhere between Banjo-Kazooie (3.6 million) and Diddy Kong Racing (4.8 million) would be my guess.