All this speculation is based on the idea that Sargeras still has his planet sized body, which obviously is impossible because if he did then Xera's forces wouldn't be able to do anything, he would just show up in Argus's orbit and WTFpwn them. He discorporated himself in order to propel his Titan Soul into pre-natal Medivh, he was so sure that his victory would be absolute that he didn't consider the consequences of failure. That's why he has to attempt this questionable reincarnate-as-Illidan plan in the first place, because if the Legion could just put him back to the height of his glory, that's what they would do.