The Burning Legion didn’t stop because Sargeras got yeeted into a chair. There are plenty of Demons still out there, including many of the Burning Legion’s leaders that are still alive, either reborn or waiting to be reborn in the Twisting Nether.
Archimonde*, Kil’jaeden*, Mannaroth, Azgalor, Kaz’rogal, Kazzak, Jarraxus, Zakuuun, Magtheridon, Kruul and Brutallus are all good examples. Not to mention the literal legions of other Demons that are still out there. Argus for example, was not fully cleansed of the Burning Legion’s forces. We took out their respawn macguffin (which is why I’ve marked two of those names with *) and killed quite a few Demons in three small areas, but there was a whole planet there full of demons that we didn’t touch, which says nothing for all the other demon controlled planets/strongholds like Niskara, Kerxan, Rancora, Xoroth and Tarratus.
Now what about that * that I put up there on Archimonde and Kil’jaeden.
Well quite simply, I’d argue that neither of them are actually dead and that both are waiting to be reborn in the Twisting Nether. Here’s why:
For Archmonde, while we fight him in the Twisting Nether in the mythic phase of his fight during Warlords of Draenor, he clearly doesn’t die until after he returns to Draenor. He is still very much alive and has enough power to zap Gul’dan, sending him to our universe (and thus creating the setup for Legion) after his defeat, and his eyes dim after he fully collapses onto the ground. An Eredar (regardless of them being Draenei or Man’ari) dies when their eyes dim, which we also saw with Vindicator Maraad when he died at Shattrath. Given that he did not die in the Twisting Nether, his soul would have been snatched up by Antorus and he would either be reborn, or be in the process of being reborn, when Antorus was disabled. So he’s still alive.
For Kil’jaeden is a bit more complicated. Prior to 7.3 and the reveal of Antorus and its powers, it was safe to assume that Kil’jaeden was 100% dead. After all, the demon lore we had at the time told us that when a demon dies in the Twisting Nether, or a place saturated with energies of the Twisting Nether (such as a demon world like Argus) then they’re gone for good. But, the reveal of Antorus changed all that, as Turalyon confirms for us that even demons that die on Argus will be reborn in Antorus as long as the facility is operational. Given that Antorus can snatch demon souls away when they would otherwise be obliterated, it’s a very safe bet that Antorus could do the same thing to demon souls that die within the Twisting Nether, and as Kil’jaeden died right above Argus, within close proximity of Antorus itself, it’s a safe bet that his soul was snatched as well.
Although that being said, if Kil’jaeden is still alive, he may not be the same demon we’ve fought before. He seemed to change after his defeat above Argus, so who knows.