You think the REMNANTS of Y'shaarj, and all of the Sha, were more dangerous than Y'shaarj himself during the Black Empire days? We 'defeated' N'zoth with the power of a world soul, so if Y'shaarj is the most powerful of them all, and his Sha were even more powerful, how was some random meathead Orc able to control such power? What is this headcanon.
Garrosh was hardly in control, he was fucking insane towards the end, the power was absolutely controlling him; just because he has lease to direct the power does not mean he was in control, he was unhinged.