And what the hell is the idea with "longer than it should"? There's no end to WoW. As long as there are playing and paying customers Blizzard will keep cranking out expansions. If there's still million subscribers left in 2025 we'll be in expansion twelve by then.
I wonder why some people seem to think that the 'story' of WoW has to end, or that the 'plot' can't truly advance without a WC4 or a WoW2 to push it forward. The world is never going to 'live happily ever after'; there'll always be something happening next. Whether or not the writers of the game can make what's next be interesting and new, instead of stale, is the only real limit on the game's longevity.
I imagine some alien race in a higher dimension playing World of Earth, going, "What do you mean the WW2 expansion is all about German aggression? We spent all of the WW1 expansion fighting the Germans, can't the writers come up with something original?"