I do think they should. The only thing that is worth keeping prestige is the achievment and titles, imo.
I got cutting edge titles from mythic raiding, and transmog from that. I don’t feel devaluated that people can go into the same raid I progressed a whole season to do, 2 expansions later, one shot the bosses, to get the same mount and transmog as I did. Since I already have the title/achievment.
make elite sets and mounts comeback, but at a challenge, f.example. win x amount of games at 2400 rating (same as you needed before to get those mogs/mounts) and you get a token to purchase either from each old set.