Ahoy there @gianni001!
I think your argument of the time limited part and exclusive cosmetics being mostly for attracting new players to the game and thus being a money grab seems the weakest possible explaination for it. As @SurveyorPete also stated below, there won't be anyone going "Oh wow that's some nice looking sails, I should really buy Sea of Thieves for that". Sure the events will probably attract new players, however that's just the effect of marketing, not the time limited aspect or the cosmetics.
I can think of several valid reasons why the time limited aspect is a good things:
Focus on the event
Each of the events introduced content that is hard to beat by yourself, so having people around that are all focused and know about the content that's currently live will make it easier to find people to do it. Removing that focus will be like trying to find people to do the Thrones now.
Focused feedback on the new content
Apart from focus for the players it'll also give focus for the developers as they can see how everyone is interacting with the newly added content and they can, like they did with Cursed Sails, make adjustments during the event based on a lot of feedback.
Information focus
By keeping it time limited all the information about the campaign can be focused within that time. Everyone will be talking about the campaign, all the loading screens can give information about the event, all the communication channels can focus on it. This way everyone playing the game will know what's up and everyone not playing the game might be tempted to get in.
Player activity
Somewhat similar as the player focus, it'll also make sure that people who log into the game once every month will maybe try and plan their time to actually be there during the event. It'll increase player activity for the limited time.
Introduction/lore for added content
Apart from the actual added content, the campaign is meant as a introduction for content that's being added after the campaign. They could have just added the Megalodon or the Skeleton ships to the game without any introduction, smack some commendations on it and let everyone do their thing. However they introduce it with an event before adding it to the game.
Renewed interest to come back to the game
Unrelated to your money grab theory, there's a lot of people that might have bought the game but stopped for whatever reason. By making the event a short term thing with the dates being added to the trailers, you can get people to come back within that time.
Rewarding people for being at special events
As you said the game is literally about cosmetics, so they're used as a reward mechanism. People are rewarded with cosmetics for completing things and for playing the game at certain times. You can see by the cosmetics someone is wearing what events they were able to complete. Sure this can be a though pill to swallow for people that joined later and cannot get it, however that's how it works with a lot of things and even a lot of games. On that topic though, Rare did state that the timed limited items might make their return at a different time, so that doesn't mean you'll never be able to get them again.
Having exclusive cosmetics in a game about cosmetics doesn't sound like a bad thing to me especially not if everyone that was playing had the same chance of getting something. It's not only a reward for accomplishing something special, it's a the developer rewarding content for people being there and supporting them.
The only problem I can see with it is that a certain set of cosmetics is spread out over several events. I don't really mind the ship / weapon cosmetics being seperate, however to complete the bone crusher clothing set you need to have done the Thrones event and The Cursed Sails. People that are tempted by starting the game again will find out during this event that they missed out on parts of it. I think it's weird to split up clothing items in such a way and although I was able to get all of it, I think it would have been better to keep clothing sets together.
That's my 2c about it..
Signed,
Captain FishSt1ck
[Updated: My post was written just when I woke up, I though I'd add "a bit" more]