In a vacuum, Sea of Thieves is an excellent game. It offers an experience unlike any other, giving people a chance to imagine themselves and their friends as pirates in an open world filled with hints of magic and mysticism. Indeed, the first couple of hours of Sea of Thieves are great but, unfortunately enough for Rare, the game does not exist in a vacuum. The general consensus is that while the game has potential, its lack of meaningful or varied content torpedo its longevity. One can even argue that if it weren't for the existence of other, similar games in the past, Sea of Thieves could have set a new standard, but alas other games have shown what open-world games are capable of.
Take, for example, Assassin's Creed IV: Black Flag, the game that most people think of whenever any game involving piracy is mentioned. There are NPC controlled convoys to raid, you can board ships, and you can fight legendary ships and capture forts. By comparison, Sea of Thieves has no NPC ships whatsoever and the skeleton fort raids and kraken encounters are rare occurrences. As far as open-world multiplayer experiences are concerned, GTA V offers all kinds of missions, and while they mostly boil down to going to some place and killing some guys, the actual objectives are varied enough that most people evidently don't notice or care. Meanwhile, Sea of Thieves tasks you with sailing from one point on a map, picking something up, and then sailing back, and that's about it. The list of comparisons can go on, but at the end of the day, Sea of Thieves simply cannot compete with the depth of gameplay that other games offer, which has proven to be a source of major disappointment.
Fair or not, such comparisons ultimately illustrate the state of gaming nowadays. People expect open world games to have missions or quests or whatnot around every corner; people want end-game activities to be these skill-intensive but fair encounters that can take an hour or more to beat; and people certainly want to feel and see actual progress when they play, regardless of whether it be by having a more effective character after hours of play or by an improved sense of personal skill. Failing all of that, a game should offer an interesting story at the very least, but Sea of Thieves doesn't have any noteworthy storytelling element whatsoever.
It may be a bit of an exaggeration, but Sea of Thieves certainly gives the impression that it is but a devolved anomaly in the long lineage of game design. There's fewer NPCs, less story, less variation in content, less of virtually everything that its competitors would and should strive for more of, and that's a huge problem as far as public perception goes. No one expects any game to push the boundaries of every single facet of gaming immediately upon release, but people do expect a new game to build upon the successes and failures of previous games. That is what progress is after all, and without such progress, gaming may as well consist of millions of variations of Pong and nothing more.
In all fairness, adding features to a game takes time and resources. Adding NPC controlled ships to Sea of Thieves would require someone to figure out where the ships spawn, how the ships interact with players, how would they interact with the environment, how often do they spawn, and so on and so forth, and that's just the bare minimum. The problem with that is that for the end user, they likely wouldn't care about anything other than their own experience, and it's hard to blame them. It would be like going to a restaurant, ordering food, and then finding out that half the food you ordered is going to be coming after a two hour delay. You wouldn't care why your food is coming late, you would likely just ask for a refund and leave. There is little doubt that Rare blew the best chance that they had to impress people as word has undoubtedly reached the ears of many that Sea of Thieves has a critical content drought. Even if Rare has some amazing new features planned for the game, your average consumer is just going to go to another product because there's so many different, high quality options available right now.
Naturally, some will say that Sea of Thieves is fine, and that claims of its demise are greatly exaggerated. For them, the game is fun, and having fun is all that matters in a game. To a degree, that is true (to each their own after all), but complaints of the game's lack of content are not isolated, and they're certainly not unwarranted. People may be playing the game today, next week, maybe even next month, but what about the month after that? How about when Red Dead Redemption 2 comes out? Rare has to address this question if they want Sea of Thieves to be more than a flash in the pan, and right now, all signs point to a singular solution: more meaningful content. Rare is already at a huge disadvantage given how Sea of Thieves' current predicament is so similar to a number of spectacular failures that are still painfully etched in recent memory, but they must act fast if they want to salvage the situation as people (perhaps rather desperately) want the game to succeed.