The Legion isn’t really ‘defeated’ though. I’d say stalled more than anything else.
With Sargeras imprisoned and watched over by Illidan and the other Titans of the Pantheon the Burning Legion is temporarily leaderless, but there are many high-ranking demons who, thanks to Antorus, are still very much alive or in the process of returning to life.
In fact, thanks to Antorus, every major demon leader that we killed prior to shutting it down is still very much alive. Even the ones that were killed in the Twisting Nether.
Why do I say this? Well, because of how the lore surrounding demons works, we know there’s only two ways to permanently kill a demon. To either kill them within the Twisting Nether itself, or to kill them on a planet saturated by the energies of the Twisting Nether. However, while we’re on Argus, Turalyon lets slip that one of the Lieutenants of the Burning Legion that we slay will simply be ‘reborn in Antorus’. This creates an issue, because Argus is a planet saturated by the energies of the Twisting Nether, considering just how corrupted it is.
So if Antorus can snatch a demon soul that -should- have died for good and preserve it, and distance is not an obstacle (Varimathras’s soul was snatched up despite the fact that Azeroth was thousands of light years away from Argus at the time of his death) that means every demon we slew is still kicking around somewhere. We may not see them for a long time, but they’re still alive.
And as long as they’re alive, eventually the Legion will return because they’ll want to finish what they started.
That means demons like Archimonde, Mannoroth, Doom Lord Kazzak, Tichondrius (although as we now know, he and the other Dreadlords are loyal to Sire Denathrius), Azgalor, Lord Jarraxus, Brutallus, Magtheridon, etc, are all very much either alive or waiting to respawn.