It's not like most stat gear doesn't eventually become irrelevant most of the time.
And you shouldn't be so sure. FFXIV provides clear proof that people will chase after vanity items and often times make lots of money selling them. I do The Feast (pvp) in FFXIV, which has a rare chance of dropping a Fenrir Pup minion to you after every match. I've made millions of gil selling them (to people who won't do pvp) even though it came out years ago. So, no, vanity items don't have to "become irrelevant in about 6 months."
In FFXI, most vanity items have historically come from campaigns and events. So we honestly could do with some that are earnable through regularly available content.