I understand the $60 price tag. I really do. In terms of work hours, this game will probably end up with more than most full priced games. It might already be there, assuming they've been working on additional content as soon as they finished the "base game". They probably have a lot of updates queued and ready to test as soon as they're done with initial bug fixes and patches.
However, that's a pretty tough sell to consumers. A lot of people don't want to drop $60 on promises.
On the other hand, if they're pushing "games as a service" then they won't really care about that as much. The key there, of course, will be continued exposure. Expect to see a continued "Sea of Thieves" presence during Xbox conferences in the same way that Bethesda always talks about Elder Scrolls Online. They want people to bite on that subscription. It's okay if it launches as a simple shell of a game as long as they keep adding content, which creates the impression that it's a great deal for the consumer. Perception is of the utmost importance.
Why is a subscription so valuable? It's a rock steady revenue stream. It looks small but it builds up rapidly, especially considering the "it's only a few dollars a month lol" way some people think.
I recently had a coworker tell me that they've been an Xbox Live member for like 10 years. Not all that spectacular, really, but then they laughed and said they even went a few years of that without even owning an Xbox console. They just let it ride on their parent's credit card.
People like that are the reason why subscription models are so attractive.
