Blizzard has adjusted their release strategy for a number of reasons.
They have, no doubt, seen that they're game has lacking content for veteran players, you can only reroll and redo all the quests so many times before the game becomes a drag.
Narrowing the release window keeps people saturated with new content, that's their aim.
EVE Online is a prime example, it doesn't have the motherlode of subscribers that WoW has but their release windows is approx. two expansions each year (in content they're smaller than WoW's).
Adding new content systematically keeps people entertained and saturated.
With 10 million subscribers WoW is far from dying, it might've had it drop but that can also be accounted to the new MMO's being released, people can't subscribe to five MMO's just to try them out, they'll have to drop one. And WoW would be the easiest choice since you'll likely have seen all of the content it has anyway.