WoW has long been the biggest but they never had anything approaching 90 million subscriptions. At the height of WoW's popularity they hit a high of 12 million subscriptions. So, they have at this time lost a number approaching half their player base which while significant certainly leaves a lot of paying players.
WoW as far as I know remains one of the few MMOs (are there any others left?) that can get away with requiring a monthly fee.
WoW has a long way to go with so many users before it stops being profitable that is for sure. Consider that games like Asheron's Call, Dark Age of Camelot and Ultima Online and the original EverQuest (which is doing the best of these) are all still up and running. Even Vanguard which by all accounts was a colossal failure is still running so they must have enough users to be making some profit.
WoW tends to enjoy an influx of returning as well as some new players whenever they release another expansion too. About the only thing that might be able to really cannibalize WoW would be Titan and that's a big maybe because it depends on how similar or different the game is. Plus it is amazing how invested people become in their characters in virtual worlds.
I'm still enjoying WoW myself and expect that I will be on and off for a long time. I've invested a lot in lining my ducks up in a row (the alt army) and it will be fun to level the various classes now that money is no problem and all are decked out in full heirlooms. And that is just on the Horde side. I'm thinking I just may do it all again on the Alliance side as well. My guild has chapter guilds on both sides and I'm in both of them. I'm looking at many hundreds of hours of gameplay there easily and on top of that I like single player games...
Most likely my real bad. I saw that 90 million somewhere online. :-\