People who don't do Mythic, raiding or PvP:
They can revisit backlog of 18years of game production - fully accessible -
They can farm tmog and mounts.
They can level alts.
They can do achievements.
They can do pet battles.
They can do RP.
Etc.
TBH - I have never played a game, where I burned out - not because of current, but legacy content. There is so much stuff to do.