PC Gamer posted an interview with Tom Chilton where he discussed the possibility of World of Warcraft eventually going free-to-play. He clarified that there are no active plans for this now, but he can imagine a future where it would be possible.
As massively multiplayer games become more mainstream, a common tactic to compete with WoW is to make the games free to play while charging microtransactions for in-game content. It makes games more accessible, and in theory can make publishers enough revenues that they can be profitable. If enough games break even this way and WoW starts losing momentum and market penetration, instead of shutting down, it would only make sense for Blizzard to keep the game available for the residual income. The original EverQuest is still live, for example. Instead of leaving vast empty realms populated by a very small number of nostalgic gamers, however, making WoW free-to-play could keep it residually profitable for longer.
While WoW will undoubtedly lose the majority of the market share some day, this day is not close. Warcraft is past the red-hot growth phase of its life cycle; however, it's still the largest player in the market. Chilton acknowledges that there's no reason for Blizzard to make this change while this is the case: "We're not spending a lot of time thinking about it. It's not something that's a reality for us in the near future."