Unfortunately for some players, very likely not. While Kil'jaeden was definitely one of the driving forces of the Legion (and I'm sure we'll see more in the lore cutscene that apparently takes places at the end of his fight, whenever SK posts the video of it), he's nothing compared to Sargeras himself, and we'll definitely need more than a raid to topple the World of Warcraft's biggest big bad. Throw in the fact that there are still demons running rampant all over Outland (not to mention portals from all kinds of various places bringing them in), and even though we've just spent a whole expansion trying to kill the Burning Legion, odds are that we'll see them again soon.
But not too soon -- hopefully, as Morbosa says, the next expansion will offer a return to the old swords, sorcery and politics that high fantasy lovers love so much. It's not that we don't enjoy fighting demons -- they're fun. But nothing feels as good sometimes as a mace cracking some skeleton heads towards the goal of beating a king of the undead, and hopefully that's what we'll find in Northrend.