That's good, they are just going to have to continually reevaluate their strategy. I don't know how they are doing financially, but at the end of Cata (or maybe WoTLK - I can't really remember right now), when they lost something like 1.5 or 3 million players, they still produced record profits from WoW because they made their pet store more robust. So they may be collecting less money from subscriptions, but more money from the pet store (especially now that they introduced pet based combat) which might be keeping them higher than it seems they should be considering the loss in subs.
Furthermore, I don't know which section of their population they are turning over the most, but it seems to me that if they want to continue their historic high levels of retention that they are going to need to start catering more to the players that continually resub - that might mean less daily quests and more thoughtful raids, or it could mean exactly the opposite (though I think it's going to be hard to maintain casual subs when so many games are going f2p or are being developed for the f2p market).
In the end I think the only thing that can dethrone WoW is that their developers are really in a stint and they systematically program every expansion to be identical to the last with different loot hallways, grinding spots, and item grabs. (MoP probably =) Cata = WoTLK = BC insofar as the basic premise behind the gameplay is designed, which is basically Blizzard saying "Forget everything you did before, because this shit is waaaay better." It's also noteworthy that there are typically substantial drops in subs in the midst of long content patches (especially content patches with very little content). We saw tons of people quit in WoTLK for ToTGC before ICC and we saw the same towards the end of t11 and t12 in Cata.
As far as new games that might take some of the population? I can only think of W* and the next WoW expansion pack. The former I say only because it seems to be pretty much a copy/paste of WoW (with a different lore) in terms of content development and art design, but it overlays that with GW2/TERA-esque action based combat. Some people who play WoW may want to move to action combat after forays into either GW2 or TERA (or MO or DF:UW or DCUO, etc.). That said, I think the biggest threat to WoW numbers is their next expansion. People quit early towards the end of their current expansion once they kill the final boss because all of their efforts of obtaining more and better gear beyond that point are invalidated within 1-2 levels (less in Cata and MoP because of 5 levels instead of 10) of the next expansion (which are balanced on the guys in blues and greens, not the guys in the shiniest epics, which means the gear doesn't even matter because you would have blown through the content anyways), and they know there will be little to no content produced for the rest of the expansion they are in.
In fact, one of the reasons why I'm considering resubbing WoW now is because I will log into the game and literally have 3 tiers of raiding to go through with if I don't skip content via RF (which I inexorably will, of course, and which makes me second guess going back in the first place).
DAoC > WoW
Call me sir.