Blizzard games ALWAYS take long.
Yes, BUT:
2004-2011:
-World of Warcraft + Burning Crusade, Wrath of the Lich King, and Cataclysm
-1/3 of Starcraft II
1996-2003
Warcraft II: Beyond the Dark Portal
Diablo
Starcraft + Brood War
Warcraft II: Battle.net Edition
Diablo II + Lords of Destruction
Warcraft III + The Frozen Throne
As an MMO, WoW requires more resources and more content. The focus is understandable and logical, given how much money it generates. I'm not privy to inside information but it seems like WoW is slowing down. Realm pops are going down and people are moving on (Rift). Still it's the biggest MMO and will rebound as more content is released.
Diablo has less financial potential than WoW or StarCraft; with SC2, Blizz took a gamble by catering the game more for western audiences than the Korean scene, and it has paid off significantly with the game selling well worldwide and dominating the western e-sports scene. Who knows, maybe Blizz is making the pvp/multiplayer portion of D3 more significant, which would help the game grow beyond just being a single player title with niche competitive play. WoW's pvp scene is dying afterall
Regardless, D3 is going to sell gangbusters for years like all other Blizzard games. Is it the company's #1 priority? Of course not. But that doesn't mean it's not a priority. I'd imagine Blizzard is focusing on their new MMO, SC2, and making WoW last longer