MS wanting you to subscribe to GP for this game isn't hanging SoT out to dry! MS is shifting their entire ecosystem to this Netflix sub model. When Halo 6 launches on GP, will they be hanging that out to dry as well? Of course not. MS wants you to opt into their ecosystem for a impulse buy price of $10.00. Then when you are in, they give you a ton of content (100X more than your $10.00 should afford) and slowly trickle in exciting new content every other month or so, to retain you. MS would be thrilled if SoT didn't "sell" a single copy and instead they got 2 million new subs for GP right as SoT launched into the service. Eventually we may see MS just launch games exclusively on GP. That is the beating heart of the Xbox business model going forward: affordable subscription for content and services.
As far as the game being content bare, I share that concern. But I think the "bones" of the game are so solid and rich (the boating and water simulation in particular are stunningly well done) that you can make your own fun. I know some gamers will roll their eyes at that, but it's true. One quick example: I got on with my brother-in-law and we tried and failed to kill a full Ghaleon crew 3 times. We stole their boat and almost got away once, etc. So we were making our own sea battles. I was swimming to their ship, sniping their sailors on deck as sharks were coming at me. Attacking a defending sailor underwater with my knife, etc. We had a lot of fun, even though none of this was "content" or "quests." But again, I agree, they do need to shore up the content and quests with more variety and combat, etc., and I'm sure they will. They are promising new content in 3 months, I think. And honestly, I haven't even sailed with a full Ghaleon crew, taken a skeleton stronghold, fought a kraken or completed more than a handful of poorly paying intro quests. And I'm having lots of fun. The game is beautiful and the art style is charming, imo.
The cool thing is, now that they have these great underlying systems and foundations, hopefully they can build upon them with all sorts of content, like fishing from the boat, scuba diving caves for treasure, visiting a mermaid lair, freeing slaves/ransoms taken from NPC pirates, and all the other cool things others have mentioned.