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To start, I already pre-purchased GW2 and I like what I've seen so far.
But, I've seen many instances of when people talking about "Maybe this game will be the one that kills WoW or Tera or SWTOR or whatever". No, it will not happen. No game kills another game. Ever. Period.
1. To each their own.
Only those who have already quit WoW, or have already been planning to quit, will switch over to GW2. If someone still enjoys playing WoW, why would they switch to GW2 and stop playing WoW? If they enjoy both then they will just play both. No one quits WoW when another game releases unless they already stopped enjoying WoW.
2. Games only kill themselves.
I cannot imagine any successful game in history that died because another game released that was more popular, unless it was made by the same publisher (and therefore they're not competitors anyways). Every game that is dying or has died is because of itself, whether the mechanics are out of date, or the publishers pissed on the players. So the only way WoW will die is when its population slowly dwindles to a very low state, or if Blizzard start shoving poop down their players throads. Not because of another game releasing.
So did Rift/SWTOR/TERA etc.etc. kill WoW? No. Are they successful games? Sure. If some population of WoW switched over to those games, it's because they were going to quit anyways. These people weren't "stolen" from WoW. Blizzard would have lost those subs regardless of those games releasing.
So, please stop saying things like "GW2 will kill WoW", or any game killing any other game. It won't happen. You will look like a jerk and you will be embarrassed when it doesn't happen, like it never does.