@mr-whaletoes
You could then sell it for gold like any item

I could see people not liking this idea for the whole risk/reward for super rare items, especially if you're not good at the game, like imagine getting the Shrouded Ghost, but you're new to the game and can't defend yourself, so some Legendary Sea Dog group rolls up, sinks you and get the Ghost for themselves. What are the chances you'll ever see the Shrouded Ghost again...? Very, very, low. However, a mechanic to allow players to naturally earn cosmetics would still be beneficial:
I feel like if they put enough cosmetics into a rotation and have it as a random spawn island event it could work.

Look at Deep Rock Galactic; they have a separate resource called "Matrix Cores" that are used when you find a "Core Infuser" event while out doing missions. Once you complete a Core Infuser event, a little screen pops up that allows you to insert a Matrix Core and choose 1 out of 3 options; a cosmetic, or one of two weapon upgrades (now for Sea of Thieves it could simply be divided into a weapon, clothing, or equipment cosmetic) and that is instantly applied to your account, whether you successfully complete that mission or not. The twist with these events is that they're super dangerous, or can take a long time to do, and they're all on a 2 minute timer (I think its 2 minutes, but you get the point even if they're not). This is an area that SoT could expand on easily and benefit from, allowing more "rare" cosmetics to be added WITHOUT having to rely on Twitch Drops, time limited availability, or super special ones like the BMP HAT and requiring the player to actually "win" something.

@hamanito2211
Well, eventually you get the cosmetic in the box and that specific box becomes worthless to you.