One day WoW will die. The issue here, and the issue most people can't or won't understand, is that it's not going to a single thing that kills it... unless that "thing" falls under the definition of "time." WoW is already almost 10 years old, and this strange age of the Internet and the information age, that's basically the equivalent of well past retirement age. WoW's "peak" has come and gone, and while it lasted it was it most definitely a rush to be remembered. These are WoW's twilight years - it's basically cashing in Social Security checks, getting senior discounts at Denny's, and if it hasn't already made the move it's really looking into the real estate market in Florida. But MoP hasn't killed it, and TDB/WoD/Whatever the next expansion is certainly won't kill it. Every expansion and content patch breathes new life into the game, like a dose of blood pressure medication or a vitalizing treatment at a downtown spa. But each of these, over time, will deliver less and less life... until finally WoW settles into the somehow immortal state of torpor achieved by games like EverQuest, Asheron's Call, and Dark Age of Camelot.
At that time WoW will still be around, still be a thing to be played, just by a whole lot less people. Another game will have taken the crown of "top MMO," perhaps it will be Titan, maybe it'll be something by a new publisher fresh off the presses with new ideas and new technology. I find the bickering and speculating over sub losses to be some strange theater, because we know how its going to go down. Hell, we've seen it go down before with MMO's that have preceded it.