system
Given the following post, respond as if you're commenting on Reddit:
human
[Warcraft] What if Illidan won over Arthas at the base of the Frozen Throne?
gpt
The Scourge would have wasted away, as Ner'Zhul's power continued to bleed from the breach in the Frozen Throne. Those undead which still maintained a relative conciousness would have broken free and become Forsaken. Some, certainly, would have joined Sylvanas, but most would likely have broken into their own factions or sought to end their miserable existence. 

Of course, we know that Varimathras was never loyal to Sylvanas, and without revenge on the Lich King to drive them, the Forsaken would have been far less united. Bringing the undead back under the control of the Legion would likely have been simpler, because they would have been more receptive to a new purpose.

Furthermore, Illidan was actively working to Kil'Jaedan at the time, and likely would have continued to do so afterwards. His creation of the Cask of Souls was an act of desperation to protect himself from the Legion, and without his failure he never would have done it. 

End of the day... short term, everything's better. No Plague, no threat of the Scourge... But the Legion comes out of it with more of a presence on Azeroth, and everyone dies soon afterwards.