They'll be there, but this isn't THEIR expansion. This is the expansion of Grom, Garrosh, and Kairoz (who right now is my pick for Final Boss).
Yes, the Legion doesn't go away, but the Legion also doesn't give instant gratification. When Sargeras was defeated, he didn't just summon a huge army to Azeroth right away. He put his spirit in the unborn Medivh and waited until the time was right to get revenge. When the Night Elves stopped Sargeras in the War of the Ancients, they didn't return for a LONG time. When we stopped the Legion in WC3, they didn't return in force until 2.4. And when we beat them in 2.4, they still haven't returned for vengeance.
Kil'jaedan isn't going to just let Gul'dan summon a bunch of Demons to Draenor and think that they can stop the Iron Horde PLUS the "off-world invaders." He's going to wait, he's going to consider the options, come up with a viable one. Quite likely he'll focus on corrupting a different race to use in conjecture with the few "Fel Orcs" that exist. He'll return to take vengeance, and we'll have to stop him. But the question is when, it's a dangling lore thread more than a hint at the final boss/raid.