I don't think arch demons are that different in power. The 5th Blight was ended so quickly because of the tremendous luck of our PC (and gameplay/story mechanics), not because Urthemiel was so much weaker than the previous ones. I pretty sure that when Razikale and Lusacan arise, whether as arch demons or just regular old gods, they are going to be no pushover. Dumat may have been superior to the others because it would seem he was head of the pantheon but the deadly Void magic described in the Last Flight would be available to any of them.
I'm pretty sure that as Grey Wardens seem willing to use any means necessary to stop the Blight, if it had been possible to train a cavalry of wyverns they would have done so.
The old gods are not equal in power,the more they are powerful the more stronger is their calling and it is not really a coincidence that the first to be found was Dumat and that him and Zazikale endured much longer than the others.
Each time an old god is awakened into the corrupted form it require a lesser amount of time in order to be killed compared to the previous one without any significant improvement in terms of technology by Theodosian,it is simply statistic here from the 192 years of Dumat to the 1 year of Urthemiel
DUmat and Zazikale fought against the Tevinter and Orlesian empire at the pinnacle of their power and were a great problem.
The GW are really incompetent when it comes to research and experimentation,in 1 thousands of years they never experimented with darkspawn and never discovered that they could have been awakened.