Eh I think classic will be popular. Obviously it has some big class/spec balance issues but aside from that its a solid game. You can say "muh quality of life" all you want but look at WoW today with its endless QoL improvements compared to classic, its a shell of its former self that feels like a single player game with a lobby. Also who knows what modern QoL improvements they will carry over. They already said their shareable loot system will be carried from modern. Wouldnt surprise me if they include duel specs or other similar changes.
Yeah there will be a good chunk of people who only resub for 1-2 months out of nosalgia but if EQ(a game viewed with far thicker rose tinted glasses) can continuously release multiple tlp servers every few years then no reason at all WoW's wont be at the very least as successful. Just look at the classic>wotlk emulated servers. Even with how horribly they are managed they have continued to draw big crowds over and over. But compared to a legit server where you dont have to deal with legal threats, admin favoritism, your personal info being compromised etc.? Going to attract so many more people.