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The perpetual search for the "World of Warcraft Killer" is a constant theme in these forums as well as many others, but I think I may have hit upon the secret to making it happen: allow players to migrate their existing World of Warcraft characters to the new MMO.
Of all the reasons millions of players stick with WoW, I think the most important may simply be that they have already invested so much time into the game that they are reluctant to let that "investment" go. Yes, anybody who has taken an economics class has learned about the sunk costs fallacy, but I am going to go out on a limb and guess that a significant number of players have never taken an economics class, or not taken its lessons to heart.
So make it easy for them: let them migrate over the characters they have worked so hard on, and not even have to face the possibility that they have wasted the last six years of their lives on a character in a game that is not going to be around forever.
(I was going to close this out with some smilies, but the more I think about it, the more I think it is not a joke . . . I have heard WoW players say that they would not even consider a World of Warcraft 2 unless they could migrate over their existing characters, which really says something about the psychology of it all.)
Edit: Yes, I realize this can't really be done, which is why I don't think it is going to happen, which is why I think there will be people lying on their deathbeds in fifty years worried about getting T57 armor on their lvl 785 Paladin.