People said the same about RuneScape. No-one would play on legacy servers if they had to pay for them, EoC made many things better, tc. Now 07scape brings in just as much (if not more) than 3.0, and made a LOT more the first year it was re-added.
That said, I do think there's a few groups calling out for Vanilla. First off are the people who joined any time after Jan 2007 - Even in early TBC, you couldn't escape vanilla greatness talks, so people obviously wanna see it for themselves. Then you have folk who enjoy a decent grind (and please, don't compare the T0.5 journey to world quests - One is/was epic, one is busy work).
Mostly, though, I think the folks calling for vanilla just want that old school sense of community. Your realm used to be an important thing, but now it's just a lifeless shell where you find a guild & ignore the world.
Still, I've no interest in playing on Legacy servers myself, and can't see them coming in the near future as Legion is sitting on >10m subscribers... BUT. One day, it'll happen. Just like RuneScape - When it makes business sense to do so, they'll do it.