And let's face it. Most of the big bads that people care about have been defeated already. We can make a fairly reasonable assumption from looking at sales numbers that more people played Warcraft 3 than Warcraft 2. Illidan, Kae'Thas, and Arthas all died by the end of the third expansion. Very people who hadn't played WC2 would have even had a clue who Deathwing was, or why we should care about him. So you have natural aging on top of having most major known lore characters dealt with. That sounds like a pretty natural recipe for declining subs to me.
I'm only going to start caring once it dips below 2 Mil. Anything above that and they can still justify a fairly decent staffing and development since they can always link servers to reduce their hardware and operating costs.