Classic is incredibly populated at the moment, even queues made a return. We're +7 months after Classic launch and ZG, AQ and Nax are yet to be released and people are still hooked just for MC, BWL, PvP and the "old version" of a game.
I know that people like to belittle or trash Classic but it's already proven to be a tremendous success. Based on recent numbers it might contribute a high six figure sub number to WoW which is, at this point, a lot.
Classic (or the concept of "old" servers) is here to stay, their TBC survey just proved this.