The subscription is for "World of Warcraft".
Basically there will be two world types: Classic and *insert current expansion here*
Both groups are paying for World of Warcraft as a whole, not just their specific world type.
Classic isn't going to be a separate game, simply a separate world type. (Much like how PvP & PvE servers used to have a different ruleset.)
It's even under the "World of Warcraft" tab in battle.net.