That line that Corypheus says after being released in the Legacy dlc while he's all confused "Be these some dream I wake from? Am I in dwarven lands? Why seem their roads so empty?" highlights how in his time, the Deep Roads really were the commercial and trade arteries of Thedas, and how they must have been constantly bursting full of people and merchants trading and going from one corner of Thedas to the other.
Tevinter controlled most of Thedas, but below the surface, the dwarven empire was what truly connected it, all the distant Thaigs and cities bringing lyrium and other things all over the continent, and even trading with the Chasind and the Avvar as far south as Ferelden (and according to Bianca, its possible the Deep Roads might have gone beyond Thedas itself)
And then Corypheus wakes up a thousand years later and assumes he's on the Deep Roads and is puzzled as to why it all seems so empty and lifeless. And the irony is that, even though he doesn't know it yet, thats what the Deep Roads and all the dwarven lands except for Orzammar and Kal'Sharok have become. A dead, lifeless carcass filled with darkspawn, with decaying statues, crumbling Thaigs and a few squads of the Legion of the Dead representing the fading memories of the glory of the past that once was and will never be again, and slowly fading into history like the dwarves themselves.
