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Blizzard know WoW won't last forever. That is why they are renewing their other IP's and developing a new MMO.
As for whether or not they have anything to worry about from this game, of course they don't. This game clearly isn't going to kill anything, and is targetting a specific niche market. IE: Players who like Heroes of Might and Magic.
MMO's played: Ragnarok Online (For years), WoW (for a few weeks only), Guild Wars, Lineage 2, Eve, Allods, Shattered Galaxy, 9 Dragons, City of Heroes, City of Villains, Star Trek Online (Got someone ELSE to pay for it), Champions Online (Someone else paid), Dofus, Dragonica, LOTRO, DDO and more... A LOT more. I've played good AND bad. The bad didn't last long. :P