More importantly, the Calling so far is the only thing that's preventing the darkspawn from a genocide of all live and turning Thedas into a wasteland (apart from the thinking darkspawn deciding that they'd like to spend their entire existence in the Deep Roads and slowly die out due to accidents). Darkspawn are effectively unstoppable once they launch a Crusade - winning involves the desperate hope they'll stop their massacre. That's actually a big plot-hole in DA:A - that the darkspawn spot without the Mother. That's ignoring the fact that these are rape abominations.
You end up with an immortal army of self-aware, intelligent rape abominations whose very existence poisons the land and who don't require food to survive, so have no logistics. Every member of their abominable race is a warrior, and they need a constant stream of kidnapping victims to violate to keep up their numbers.
Thinking darkspawn are an incredible threat to Thedas.
The Calling stops the darkspawn to destroy the world because darkspawn are un-intelligent creatures that only think about killing and eating. They are like animals, clever and cunning, but not really intelligent. When they are "awakened" they are not only freed from the call of the Old Gods, but they also gained intelligence, and with intelligence also came self-awareness and free will.
Yeah, there are Disciples that still acted like the normal darkspawn: evil and vicious, killing everything alive on sight (the Mother and her faction being the prime examples), but there are others who chose not to be like that. The Architect faction is an example of that: even if you killed the Architect, his faction did not retaliate, choosing instead to fight the other darkspawn and not attack the surface or Orzammar. Others became good even without the Architect influence, like the Messenger, who began to help people and actually do good stuff in Amaranthine because he wanted to, regardless if he was a darkspawn or not.
That's why the "awakening" was not a plot-hole until it was ignored in DAI, because, yes, intelligent darkspawn are dangerous, but the also are self-aware and free willed, and can choose not to be dangerous.
Also, are we sure that curing the Calling is meant to remove the taint completely (thus creating ex-Wardens en masse) or just prevent the Calling part of it so Wardens don't lose their minds and turn into ghouls?
IIRC, they did not lose the corruption. There were mages in the ranks of the Disciples, meaning they still were able to do Blight Magic, that comes from the Taint (instead of the Fade). And the Messenger was still spreading the Taint in the places he helped people. They only lose that part that connects them to the "hive mind" of the Old Gods.