To be fair, the darkspawn aren't THAT big of a threat.
All the blights so far have been stopped. Some very fast.
Compared to the reapers, the darkspawn are "meh".
So I wouldn't call it so big of a threat that EVERYTHING is justified to fight it.
I respectfully, and strongly, disagree. I see the darkspawn as a threat against the known world. The aerial capabilities of the Archdemon can no longer be dealt with by griffins. The Fifth Blight was a stroke of luck because of Riordan, because it usually takes years to end the Blights.
They are a serious threat to all life, the darkspawn corrupt everything they touch on the surface (which is why Ines was studying a plant that was rumored to grow in Blighted soil), and they populate the fallen dwarven thaigs that run the entire expanse of the continent.
The Song is mainly what keeps them at bay, and there's no telling what will happen once the final Archdemons are destroyed. If all the darkspawn come to the surface at that point - possibly billions - it'll be a never-ending Blight. I think it's better to deal with them before it gets to that point.