Eventually it will get too bloated, buggy and it will be "rewritten" or "reloaded" They will release the game with a new engine, new graphics, etc, and it will be but a shell of its former self. People will then complain (like they did with new coke), that its really not the same game anymore, and slowly they will migrate towards other games, other things, and WoW will implode on itself. I give it another 4 years. It won't fully die, but it wont have the subscriber base that it has now.
While this is always a possibility, I think it is more likely that Blizzard will stay on their course of making change slow and constant so that if anything 'breaks' the fun of the game it is not too hard to change back.
If you watch the progression of the game you will notice that every few months a major patch comes out that makes substantial changes to the gameplay; changing rules, tweeking graphics, adding content, adding new playstyles. When they change or add something that players complain enough about they quickly re-patch to remove or fix it.
So, overall I think that in 10 years we will probably still have a game called World of Warcraft that a day 1 player's warlock could still be wandering around in, but it will look and feel completly diffrent, yet somehow familiar, to the game they started playing years ago.
And yes, I'm sure they will eventually make a WoW 2 and I'm sure some people will state that it is "too different" and choose to stay in WoW 1 as a result. I expect that their WoW 2 is many, many years off though.
I don't think they will ever make a WoW2. I don't think there is any need to. They will slowly replace every scrap of code in WoW, and keep overhauling it, and upgrading it, and maybe completely replace it in one go to start over with a new code base in some expansion (probably with out even telling their players), but they will still call it World of Warcraft, it will still allow everyone to keep playing their old characters with most of their old equipment (which they will encourage you to replace by making the new equipment necessary to be successful at the new content) and it will at least feel like the WoW that we know already.
I think that only two things can kill WoW. One is if a new MMORPG that comes out and is incredibly fun to play, and that fun can last for the months it takes for the word to get out to the rest of the community. Then WoW will see a slow leaching off of its player base to this new game as players give the new game a try and find it fun, so more and more of their time is spent there, they fall behind the WoW equipment curve and find that catching up feels like work, while the new game is all shiny and new. At this point Blizzard has a chance to pull players back by making changes that help players come back and is fun, if they fail that they will slowly die, but that death will take years and might be put on life support if that new shiny game makes a mistake and upsets its players. We have already seen this start to happen with Age of Conan and Warhammer, both games were fun to play at first and started to leach players off of WoW, but unfortunately both games were not fun when you closed into endgame, and so players returned to WoW.
The second thing that can kill WoW is player attrition. As time goes on players will fall off simply because life moves on and you dont have 20 hours a week to devote to a video game. As that happens WoW needs to be able to replace those players with a new crop of younger players. Right now Blizzard is making the game more about maintaining the old guard player base, and rushing new players to endgame. Eventually Blizzard will need to refocus on the new player experience to draw the next generation of players into the game. But, that is something I think Blizzard will be able to do when the time comes.
