At the very least this tells us that Blizzard knows that they've lost a lot of if not most of the people who were there for WoW's beginning.
I think they took this information and decided people left because the daunting scale of WoW was too much for them. So they began a process of streamlining everything.
This obviously netted them more subscribers as into Wrath they had more subscriptions than any mmo anywhere ever.
But I kinda think they took the streamlining too far and now it's biting them in the keester.
There are lots of people, who were new to the game in Wrath, for whom the game has naturally run its course in terms of holding their interest. They will leave in due time.
Meanwhile the streamlined game has alienated the older players and failed to live up to their initial experience of WoW. They too will leave (and have been leaving from the sound of things).
So right now it would seem they are coasting on a playerbase who are still getting their fix from the 'Skinner Box' effect. As it happens that effect wears off eventually. And then these people too will leave.
The question becomes then, does Blizzard seek to ensnare a newer, younger audience every few years? Riding on the effects of a cheap psychological trick until WoW runs out of positive word of mouth and runs out of steam?
Or does it seek to retain it's older players and rely on player support, support which garnered it's current subscription levels and longevity, to maintain itself til the game reaches its natural end?