The thing that will keep me from playing Classic WoW is time. I don't think it would be that difficult to sit down, level up, get into a raiding guild and start working my way through the raid tiers. But everything back in Vanilla just took too much time. Time I don't have anymore. I can sit down in Legion and do some world quests, do some Mythic+, do my raid for the night, all in the time it would take to get a quarter or half a level in Classic. It's certainly a different experience (not better or worse,) but Im not the endless time-having teenager I once was.