Originally Posted by
Snowraven
Because that would mean that it doesn't matter if we kill any demon even in the Nether, because Sargeras can simply go to an alternate world where said demon was not corrupted and corrupt him, creating a new version. Sure, maybe said version wouldn't be as strong... except that we know that the leaders of the Legion can insert into one's head, so he could insert most memories of the dead demon.
Essentially, this way, Archimode would never die, no matter what. Or Ki'jaden. Or Mannaroth. etc.
And this way, even Illidan's plan to battle Kil'jaden and Archimode in the Nether is... just delaying the Legion until they corrupt another Archimode and Kil'jaden and bring them up to speed. Essentially, this would mean that the Legion truly is unbeatable.