Obviously people join and quit. That’ll always be the case. But the important part is population growth. Vanilla grew the playerbase by 8 million. TBC added around 2 million and Wrath another 2 million beyond that. That’s when the subs finally started going in the opposite direction.
There’s a lot of reasons for that, but I don’t think it’s a coincidence that when the game radically, fundamentally shifted in its core design is when more players started leaving than staying. I could go into great detail about it, but tldr: the WoW devs confuse entitled players with casual players. So the game drives both casuals and hardcores away and just caters to spoiled brats.
But I digress. As far as Classic…it’s a true rpg. It relies on those things that this genre was founded upon: relationships, community, social interactions. Current WoW (and pretty much every mainstream mmo) put those features at the bottom of the list, favoring convenience and spoon fed single-player experiences. So Classic will provide something unique.