I did giggle at the folks who claimed pre-launch that MoP will get WoW back to 12 million. I think WoW's ship has sailed and that a decline is all that is left. I expect a massive drop after the end of the next Xpac. I do feel bad for the guys at Blizzard who are working extremely hard on the newer content. If we continue to see a steady decline, some of their best work may be missed or dismissed because of what Xpac it was in.
On a somewhat funny side, If Blizzard maintains their currently schedule goal. MoP may lose less people than other Xpacs because it will be the shortest of the Xpacs. If so, now we'll have to use fancy math in arguing which Xpac had the greatest and least losses. don't forget to carry the 2!