WoW is losing subs, SWTOR is losing subs, RIFT lost tons of subs, other MMOs are in decline too. What is happening you might be asking yourself? The fact is that the MMO industry is fragmenting. And by fragmenting I DO NOT mean shrinking. It is in fact growing. The thing is that more niche games are being released that cater to each individual type of player. The time of a single MMO completely dominating the genre is coming to an end. Instead there will be many different MMOs, that each appeal to a different type of player.
Many people left WoW because the game they've always wanted has been made by another company instead. In addition to this, new MMO players have many options, and thus are not pigeonholed into the "top" MMO. The lines have blurred and each MMO out there is a high quality product, none being completely superior to another. Each finding their own comfortable spot in the market, and their own loyal fans.
-TERA
-WoW
-GW2
-SWTOR
-RIFT
-Secret World
-Warhammer
-LOTRO
-EVE
-Lineage
-Aion
These are just a few of the many, many, many MMOs available. Whether you like Jedi, or you like Orcs, or you like hardcore play, or casual play, or PvE, or PvP. Thee is something for you. Gone are the days of there being a ruling "king" of MMOs, instead every MMO will find its own special spot and its own playerbase. This competition will foster an environment that will result in better MMOs for everyone.
The MMO industry is still growing, the pie is just more evenly split between the different MMOs.