This whole "let's just kill the mages and be done with it" aspect to this debate is getting funnier by the day for me.
As we all know, demons go after mages because they are more powerful. That's why mages can be dangerous to the population. However, if one kills all the mages, will the demons stop trying to possess people? No, because they still want to be in the Material Plane, and they still seek power. As has been proven by DA2, a demon will happily take a templar or just about anyone that is weak and/or willing to be taken.
So the next step would be slaughtering the Templars, right? They would have the next highest level of power (given their lyrium use) and the next group most likely at risk of possessions. Without the mages, the Templars would mostly fall apart and there would be a bunch of strung out confused addicts that would be easy pickings.
Say we kill off all the Templars. Guess what? You think the demons would just stop there? No, they'd seek out the common folks, the average John or Jane Doe, and continue to attempt possession. The average farmer's daughter, that isn't a mage or even has any mage blood, could be next. So what, you kill of all them too?
This is why a permanent solution like just "kill all the mages" will never work. The demons will not stop attempting possession and sowing chaos. They resent mortals and will not stop until the world belongs to them. Mage, templar...it makes little difference. Just different colored paving stones to power and one inch further to destroying/ruling the world.