What do you mean no gain in such action darkspawn are threat to humanity as i said pretty much you will have to face darkspawn sooner or later but this time organized so it is sane long-term thinking to crush you enemy before it becomes more powerful.Besides i don't talk about reducing numbers of darkspawn slightly only about destroying it one way or another.
Either leaders in thedas are self-centred or just blind. 
They are threat for everything alive, but for the most part Darkspawn stay underground and rarely go above ground expect for a few that kidnaps some people for Broodmothers and such. For the most of Thedas Darkspawn isn't a problem (as the Dwarfs points out in DA:O) until a blight happens, only the underground dwarfs fights them every single day, to destroy the Darkspawn in the deep roads it would take a ton of resources and no one country wants to commit that kind of resources, hell Tevinter and Orlais had trouble helping others during the 3rd and fourth blight, because they didn't want to get involved or because other reasons.
It would take centuries at best to eliminate the Darkspawn from the deep roads and it will cost a lot men and women and other resources. it would take all of Thedas with the Qunari to be able to clean the Darkspawn out, but would they be able to work together in order to do it? No because they can't barely do it during blights.