It is saying that darkspawn aren't truly capable of destroying the world because they would have do that already... In fact it takes 1 blood mage so he could summon army of demons and torn veil so i don't reason why demon wouldn't do same pretty much if mages weren't hunted by templars that how it would end... Abomnation will create more abomnations or start creating undead army have some time and we have army of undead or destructive monsters...
Actually, as far as threats to the entire continent go, the darkspawn present the greatest potential, that is, so long as there are still Old Gods remaining. The only difference is that it takes more time for them to become an organized horde. The first Blight lasted for generations, and the Deep Roads provides a very effective means of invading every corner of the continent. Aside from that, they are also a massive ecological disaster. Unlike mages, they aren't limited by basic needs, like food, or sleep. A single mage at the epicenter of an undead ass-plosion can be killed by anyone, whereas a tainted dragon at the center of a horde requires a specialized group, of which there are only ever a small percentage of the population, in order to slay it. Funny thing about the fifth Blight is that it was kind of dumb luck and Riordan's cojones that it was so short.