"scary" depends on your perspective. The way I look at it, the fact that they still have ~10 million customers for WoW is damn near a miracle -- this is a game that came out in 2004!
Surely they know nothing lasts forever, which is why they are working on that other MMO. I have to believe they are smart enough there that this eventual dropoff was too expected to be "scary".
Also, it isn't like WoW is the only thing they have going on. Starcraft 2 was a big hit, and Diablo 3 likely will be too.
Actually Starcraft 2 failed in its target group: South Korea, the original is still king in The Land of The Morning Calm. Blizzard screwed themselves over with the requirement of being online to play.
The original and Warcraft 3 are still very heavily played in the ubiquitous PC방 (an internet cafe but darker, allows smoking, 24 hour and focused on gaming). Recently, Starcraft 2 is getting even less love from Koreans because Blizzard either opened up the login process or merged servers which has allowed far more Chinese, Taiwanese and Japanese players onto the servers (which is affecting pings and causing more games to lag out). I saw one other guy in a 300 seat PC방 playing Starcraft 2 while I saw about 50% of people playing Starcraft 1 and Warcraft 3.
Surely they know nothing lasts forever, which is why they are working on that other MMO. I have to believe they are smart enough there that this eventual dropoff was too expected to be "scary".
Also, it isn't like WoW is the only thing they have going on. Starcraft 2 was a big hit, and Diablo 3 likely will be too.