I’m in the “let the achievements be the prestige” camp, and keep the cosmetics themselves available. Keep in mind we already have vendors for old “seasonal” or “limited-time” PvP gear appearances (not all, but most) and (some) mounts.
Speaking of which, here’s an example from last year where PvP season rewards would “no longer be earnable”.
" Additionally, it will still be possible to queue for Rated PvP based on your Season 3 rating, but titles and associated rewards will no longer be earnable. The end-of-season rankings will close when Visions of N’Zoth goes live."
Guess what we can buy with a Vicious Saddle now? This is just yet another example of bringing things back. It can and does happen.