Unfortunately for you Mages will never "disappear" and at some point you'll need the assistance of a mage. Remember, the only kind of Thedas entity that can combat demons is the Mage. Without the Mage in this time, demons would easily take over.
Not really. Steel can kill a demon as quickly as a spell; better, in fact, since Pride Demons can make themselves immune to magic.
In the Warden's Keep, you can kill Avernus, trick the demon into strengthening the Veil and then destroy it proving that while magic is useful, the smart ones would survive without it.
In fact, the only reason the Qun hasn't taken over everywhere is because of mage power. You'd do well to remember that.
And on the other hand, the only reason the Qun hasn't culled every mage is because of armies of normal people.
Or do you think the mage artillery would survive for long without actual soldiers doing every other job?