There is nothing wrong with people trying Classic. But giving Classic to people that fought tooth and nail to keep it from being made is a bad thing.
Also, people don’t level one toon to 60 and then quit playing in vanilla, that is a retail thing. Leveling takes time, and many have alts. You can check out pservers to confirm, plenty of 1-59 players out leveling.
Classic is a niche game for a small group of players loyal to it. It will not be some major thing that dwarfs retail or the second coming of the MMO age.