I have a crazy theory I came up with for fun.
(It's probably not true and just me trying to put some kind of interesting twist on Legion storyline)
What if Sargeras really is permanently dead after portal closed on him in WotA (correction: after Medivh's death). And Kil'Jaeden is the one trying to mantain the illusion that Sargeras is still alive and commanding the Burning Legion. Kil'Jaeden pulled the ultimate deception, but not on us, on his own forces - Burning Legion. We know that majority of demons joined in the Burning Legion either out of fear because of Sargeras' ultimatum or for power which he promised. Now that he's dead, Kil'Jaeden knows there's nothing to make other demons stay and fight so he saw an opportunity. He told others Sargeras will eventualy be back after his titan form is reconstitued and that he still keeps in touch with his spirit and is issuing his commands, so he declared himself the supreme commander of Burning Legion in Sargeras' absence. When Varimatharas was contacted by Sargeras in the Battle for the Undercity, it was really Kil'Jaeden pretending to be Sargeras.
Addition to this theory is another - Burning Legion, in reality, isn't as numerous & endless as the old stories tell.
It was the second part of Kil'Jaeden's brilliant deception which worked for him ever since Sargeras' demise. Since Sargeras' death, Legion didn't exapand it's numbers as fast as before, so Kil'jaeden came up with scheme to make Legion look like this endless unstoppable force, against which there's no point fighting, in effort to demoralize residents of the world even before war actually starts. As demons can reconstitue in the nether, Kil'Jaeden ordred them that as soon as they fall they come back and in the process help maintain illusion that Legion's numbers never dwell. Most of the times this tactic accomplished conquering world without actually figting as opposing side would lose all hope or joined them after minimal fighting.
Rumors about Burning Legion, the endless genocidic army have spread across universe just as Kil'Jaeden has planed. Kil'Jaeden started winning wars even before actually starting them all because fear the illusion spread throughout cosmos. Of course, Kil'jaeden bought in Saregas' ultimate goal of stopping the void lords via destroying all world souls so all this was for naught if Azeroth doesn't fall. Maybe Kil'Jaeden even thinks once he gets access to Azeroth's lifeblood he won't need to resort to lies anymore and will aquire enough power to literally mantle Sargeras' role.
At one point Army of Light learned about Kil'Jaeden's lies and that Legion can actually be fought against so they formed opposition. That explains how they managed to establish base of operations on Argus and are still fightining against Legion rather succesfully to this day. Legion is not as powerful Kil'Jaeden led whole universe to believe. When we come to Argus, we'll learn of this too and join up with the army of Light in the final push against illusion Kil'Jaeden created. It also explains how we're pushing against force, which is supposed to be tehnologicaly and nummericaly ahead of us by eons, on Broken isles.
Burning Legion is bearly holding on, and is in state of decline and shadow of it's former self since time Sargeras led them in WotA.
In Kil'jaedens eyes only way to keep the illusion alive is fighting Army of Light and us simultaneosly because Burning Legion is nothing
if the illusion of it's endlessness dissapeares. In the end his biggest lie will stretch the Burning Legion too thin, fighting wars on
multiple fronts to maintain the illusion.
Finally, the Deciever will become victim of his own deceptions which will ultimately be his downfall. In the end the only who profit from all this are Void lords, who revel in us fighting and not realizing that we only hasten the end by ignoring the real threat which is the enemy of all creation - Void.
EDIT: I mucked it up a little, it should be Sargeras is dead after Medivh dying, apologies.