I'm sure they'll drag out someone, but more than the aging platform, WoW's ultimate shortcoming is a paucity of new, quality story. One of the reasons I abandoned Cata so quickly was that the villains and story of Cata were such a let-down after Wrath.
The economic model of the MMO prevents the writers from ever simply ending the storyline, so they can move on to a new world, with a new story. More's the pity.