As far as actual reasons to keep costumes other than the fact that they're cool
1) As others have said, it wouldn't be that different from other non tradeable rares even if new users don't get them.
2) won't wreck economy
3) for me it has renewed my interest in the server beyond training combat. for some reason doing costumes makes skilling etc. more fun because it is silly.
4) provides opportunities for fun events (see camel races, costume contest, costume boxing, etc.) that could potentially add to economy/staking
5) Costumes are not so convincing that they can be used to scam or otherwise harm players
6) It is something that people are excited about and the coding is already implemented so no reason to take it away
7) I have seen it add a lot of positivity to community discussions, people seem very impressed by costumes and very happy to have them. I think seeing these rares and knowing there would be a chance to get them next halloween would be something that would keep new players enticed on the long-term.
I REST MY CASE