I think once Resikale and Lusacan get sent off to the Maker by the Wardens all the Darkspawn will go insane, the majority will kill each other and once thats done we will kill whats left then purge the corrupted parts of the Deep roads with Fire, Fire and I am forgetting something...... Oh yeah, MORE FIRE!
In all seriousness they have the largest army on Thedas. Unless the 7th blight takes place on par Vollen (which would probably do what the Chantry and Tevinter have failed to do and put the Qunari down for the count until the Wardens got there) they will pose a major problem once the last archdemon dies.
Ofc this is under the assumption that theres only 7 Old Gods. There might be other Ancient High Dragons Slumbering beneath the earth that No one has any clue about that once the 7 are dead the Darkspawn will begin digging towards to wake up to begin even more Blights.