There are a few things that can lead to a downward turn in WoW's numbers:
1). The recession: With the global economy in a downward spiral, people are going to have less disposable income. With less disposable income, families are going to start cutting unnecessary or frivilous expenses...and nothing says "unnecessary or frivilous" like World of Warcraft. A lot of the younger players, those who do not pay for subscriptions with their own funds, will be dropping like flies.