It took 12 years for Starcraft 2 to get a sequel (Brood Wars excluded, it was just an expansion).
World of Warcraft is still one of Blizzard's most profitable games, generating more cash/year than all of Diablo 3's sales and probably more than any gaming company makes in a year, maybe EA excluded.
Think Blizzard would be considering recreating in in a new engine with new graphics and everything, or are they laying all their future MMO creative effort into Titan?
WoW is 10 years old now, in 2 years it will be the longest living Blizzard game without a sequel (no, it won't be dead in 2 years).