Not gonna go the cross-examination route as if this was the Depp/Heard trial. You get many cosmetics by simply playing the game. You don't necessarily need to be hunting for them to pile them up. There's also a difference between looking for a specific one—I love some, and I'm happy to have them—and wanting to 100% the game by collecting every single one, which is often the issue with people complaining about time-limited stuff.
Some may disagree, and that's fine, but the mere concept of wanting to 100% a videogame like Sea of Thieves seems ridiculous to me. It goes against the very nature of a living, evolving world. Instead of wanting to be part of a fantasy, it turns the experience into a mechanical grind, and chances are people with tendency to fall into that would be happier if they changed their mindset. Again, no need to agree, but I personally believe that.
We could also go into specific cases with this, as in, does it make sense that stuff like the Spartan Set is time-limited? All I did to get that was play on a specific date, and it's something that should be available to any Halo fan. It has no value for me, I'll never use it, but it would for someone who enjoys those games and wants others to know. In those cases, I support making items available for everyone. But I think it's a common sense sort of thing. For in-game world events, unique feats associated with a season and such, I think it makes perfect sense to not re-release the cosmetics. If eventually they do re-release everything, it won't terribly bother me, but as of right now I think the people asking to keep those kind of items exclusive make more sense than those asking for everything everywhere all at once.