Is there any hope for Sargeras to become his old self again?

[Elfezen]

Sargeras and the burning legion destroy everything so that the Void Lords can't take control of it, if he had even a bit of hope that the Void can be defeated he would surely understand and try to fight them at least

[Bosen]

I think Sargeras thinks his way is the only way. There'd have to be some big discovery to change his mind.

[Soeroah]

Seems unlikely. His first reaction upon being told that Azeroth may be able to defeat them was "gotta kill it so the Void Lords can't corrupt it".

He thinks the only things stronger than him are the Void Lords themselves, and doesn't believe in another way. The only thing that'd change his mind is being defeated in combat, assuming the Fel hasn't corrupted him permanently.