Lineage 2 probably has the worst development team ever... that game was the bomb during the original release but after that it really started to suck... and people are still "playing" it... and by "playing" I mean "botting 24/7 for a week to do 3 or 4 hours of manual PvP, lose a ton of XP, and be forced to bot 24/7 for another week".
Anyway, eventually Blizz will decide that Warcraft has been played out and people will start playing other MMOs while Blizz attempts to develop another equally enormous cash cow. I mean, essentially all there is left in the Warcraft universe to defeat is Arthas aka "the Lich King" and Sargeras, the demon who is considered to be the root of all evil in the Warcraft universe, who is "missing" until they find some clever way to bring him back.
I think you might see 1 more WoW expansion here in 2 or 3 years when WotLK has gets up to v3.4 or v3.5 if there is still enough interest, but it's much more likely that people will start to demand such enormous changes that it will take an entirely new game to feed the need of the fan base.
I, for one, am really looking forward to Diablo III's multiplayer.