It depends on how you define win or lose. Blizzard has a very loyal fanbase, and the type of player to play WoW is probably the type to not play much else or look elsewhere (it's not like they could afford to buy anything else with the subscription fee anyway). So really, Blizzard only loses subscribers when they get bored, not when another game gets released.
As such, at most Guild Wars 2 could get enough of a following that it's the preferred choice over World of Warcraft for people choosing an MMORPG to get into, and World of Warcraft won't lose as a result, but if Guild Wars 2 consistently brings in new players then that's a win for it.
Also, keep in mind the different revenue models. The more people picking up the game, the more money ArenaNet makes, whereas the more people sticking to the game, the more money Blizzard makes. Blizzard can win by keeping its userbase steady, or slowly climbing up to 13 million and then 14 million users. Guild Wars 2 could win by shipping 10 million copies in the first year (although that's probably stretching it).
This might seem like I'm calling it a tie, but it's not necessarily that either, because we won't really be able to tell. GW2 is doing a good transition from GW, so it'll probably be the first sequel MMORPG to be successful, but WoW has way too much inertia to be knocked off.
As such, at most Guild Wars 2 could get enough of a following that it's the preferred choice over World of Warcraft for people choosing an MMORPG to get into, and World of Warcraft won't lose as a result, but if Guild Wars 2 consistently brings in new players then that's a win for it.
Also, keep in mind the different revenue models. The more people picking up the game, the more money ArenaNet makes, whereas the more people sticking to the game, the more money Blizzard makes. Blizzard can win by keeping its userbase steady, or slowly climbing up to 13 million and then 14 million users. Guild Wars 2 could win by shipping 10 million copies in the first year (although that's probably stretching it).
This might seem like I'm calling it a tie, but it's not necessarily that either, because we won't really be able to tell. GW2 is doing a good transition from GW, so it'll probably be the first sequel MMORPG to be successful, but WoW has way too much inertia to be knocked off.