They didnt know about their hunger before Illidan explained it, and they would never find a way to feed their hunger. Plus now, when they have the hunger, and thus are weaker, they would think thwice before leaving. Plus in some time the New Allaince wold either get reinforcements from other parts of the world and human kingdoms, or escape death and go somewhere else. Anyway the blood elves wouldnt be hostile to Alliance, and Forsaken would never get power. So if Garithos wasnt an idiot, the situation in the world would be much better.