"Kids! Stop playing WoW please. It's time for your hover board practice!"
I'm worried thats what I'll be saying 15 years from now. WoW has massive longevity potential. If Blizzard keep adding new features as well as new content, the game can only keep going up. When the graphics start looking old, they can just bring out a patch with high res textures and higher polygon models. Honestly, WoW's only real threat is itself. The main reason players leave is because they're burnt out from ridiculous raiding commitments or are fed up with the grind. If Blizzard manage to solve this problem, I dare say that even I might go back.
My mum always told me to make sure I was the first person to go into a job interview. That way the employer compares everyone else to you. WoW has a massive head start on all future MMORPG competition and as a result gamers will be comparing all new MMORPGs to WoW. Unless the new game has topless female elves that shit gold bars, there is no way WoW can be beaten. In fact I believe the only thing that can bring WoW down is Blizzard itself. Either Blizzard will decide to make another MMO (highly unlikely) or they stop supporting WoW (even more unlikely). I have high hopes for Guild Wars 2, but I can't see it toppling the giant unless Blizzard make a mistake.
Innovative game design ideas are slowing down and as a result players are getting picky with their games. Buzz features are much less popular now than good old game polish. Cool features will attract the initial crowd, but it's the polish that will keep them there. Since the MMO business world is all about retention, only the most polished games will succeed, and WoW positively shines with polish. The annoying thing is that if a new game really does have an awesome new feature that could actually threaten WoW, then Blizzard can just add it to WoW. Boom head shot.
I've got a bottle of champagne in the fridge ready for when WoW falls, but I'm sure it'll go off long before WoW does.