Hawke failed against the Qunari? Hawke kept them placated for far longer than anyone could have imagined or even expected. It was the the act of a Rogue Mother, whom Hawke had no way of controlling, that resulted in the Qunari finally snapping. Hawke manages to fix that situation by fighting off the Qunari's forces and stopping the Arishok from tearing Kirkwall apart which is what the Arishok wanted to do. Furthermore, it wasn't Hawke's duty, as Hawke was only some Noble at that point, to deal with the Qunari but Hawke did anyway which other characters make note that Hawke gets involved in life and death situations for others, and the good of Kirkwall, even when Hawke didn't have to.
Hawke also puts a stop to the murders and kidnappings. Sure, Hawke lost his/her mother in the ordeal but the fact is that the murderer is no longer a threat to Kirkwall because of Hawke.
If Hawke sides with the Mages it does leave Kirkwall in a mess, as the Templars would take direct control, but if Hawke sides with Kirkwall they can finally set out and do what's best for Kirkwall without red tape and actually deal with those like a Rogue Mother before they cause all heck.