I'm pretty sure the Blight has Ontological Inertia. Just because you kill a daimon that commanded the latest batch of daimons doesn't mean he was the one who was the source of all the Blight. In fact, it's even more likely that Overlord isn't really that big a deal. Even if he is one of the "elders", he's only one of them. And he certainly isn't as big as whatever is the source of the Blight.
Unless, of course, Tom decides to do that, since it has always really been manually controlled or something anyway.
