Vanilla does not offer me jack shit that I'd want back. Level grinding was boring and easier than in Everquest. The raid bosses have few mechanics to keep me engaged in the fight, most class 'rotations' are a single button or two. I wouldn't be "allowed" to play my favorite spec in a raid.
What does Legacy offer me that I can't get in retail? Sitting in trade for an hour trying to find a group? Woooo damn! People talking in party chat? I get that in Mythic+ dungeons. Mobs with interesting abilities to deal with? Oh wait that is just Legion.