From what I understand, demons (and those with enough demonic essence like Illidan and the PC DH) would still regenerate in the Twisting Nether as long as they weren’t killed there; it would just take a long time. What Argus did was supercharge the whole process so it took much less time for that to happen.
I thought they only regenerated because of Argus’ power? Idk anymore. Idk what’s up with demons at this point.
Nah.
Back in Chronicle, they told us that when Sargeras was working for the Titans, he realized that he was killing the same demons over and over again, and that’s when he discovered that Demons regenerate in the Nether when killed in the Mortal Realm. That’s why Sargeras created Mardum, the prison world.
That was long before Sargeras went bad. Much later than that, Sargeras went bad, shattered Mardum, created the Legion, found and corrupted Argus and exploited his World-Soul.