I didn't say humanity was the last hope against the CA. I said at the moment, being the one we know are opposing the Ca, they are the one that would result in a break off of these two races if they gain the advantage. Morat and Shas aren't undyingly loyal to the EI. One is a warmonger species actively looking for a sign of weakness because they want to fight and the other is hyper opportunistic. So if Humanity, in the current conflict, were to gain a major advantage, it could destabilize the CA into infighting. Especially considering that in lore, even the Tohaa don't know why the EI is fighting humanity like it is. It doesn't match its usual strategy. According to a section in the RPG book, the EI may view humanity as a threat.