How many people keep playing the game they're playing because that is where they've invested their time? That is where they have their level Awesome Class of Awesome with a full Set of Awesome and the meta-achievement of Awesome while having also gotten the rare items of Awesome during the limited time event. I don't know, but I suspect someone with a character that awesome isn't eager to start over wearing dinner plates glued to rags as their new 'plate armor', even if the new world is pretty awesome.
There are no WoW killers, but WoW is an every other MMO abortionist. It refuses to let go of its players tied to their awesome characters and possibly a few friends. So all the new MMOs die.
Enter the marketing department. Figure out the approximate equivalent in their own world for class, levels, and armor, then offer that to players from other games. Let them stomp in feeling awesome, and awesome in a whole new world. That might draw out some veterans, and keep them.
But there's still that problem of social ties. People don't want to leave their friends behind, but not everyone can afford to spend the typical $50 for a box on a whim. Waiting to see what others think of it is a sure way to split a group and leave the pioneers wandering in a lonely world. So bring the friends with. Give a $1 discount for each friend they sign up, counting them as a group, so if you bring a group of ten friends (eleven total), everyone gets $10 off. Saving $110 around is essentially two free boxes, a trade which the friends could sort out among themselves, plus change, but for the company it is still more sales than it would have had otherwise. Friend stick together, the company gets more money, everyone wins. Well, except the previous game, but too bad for them.
Would you be more likely to try, and stick with, a new game if it offered you a higher level character? How about if it encouraged you to bring with your friends? I think I would.