Not rly we don't know how tevinter was dealing with mages or even if demons were threat then or even how things were working...
Yeah first tevinter and their march on black city yeah thanks for blight as***** 
I've asked you this before and you've never once actually addressed it, which makes me think that you never bothered to think it through. So I'mm ask it again, and hope you'll address it for once.
One of your points has been to acknowledge the rather critical necessity of mages for the Joiining ritual that creates Grey Wardens. You have also acknowledged that mages in general are useful against the darkspawn.
Knowing that mages are rare, and that there does NOT appear to be a purely genetic component to being born a mage, how feasible do you think it would actually be to implement a generally wholesale policy of killing all but a small handful of existing mages, and extending the same not-quite-total policy of genocide against all new mages as they are found?
I seriously don't think you've thought that through at all. If mages are not just handy, but absolutely necessary, for fighting Blights, then just how realistic would it be to kill most mages while maintaining a small set number for that circumstance? Mages tend to have limited lifespans just like everyone else, and they are an extremely small percentage of the total population, with *no* way to predict when and to whom they will be born. Just how exactly do you plan to implement a policy of killing nearly all of them but keeping a certain few on hand for maintaining the ability to even create the Joining ritual at all, and on maintaining this adequate population at all times, given that there is also no way to know when the next Blight will occur, or when there will simply be an exceptionally large darkspawn raid? Or any time that the Qunari might happen to attack, given that mages are undeniably helpful in repelling Qunari assaults?
And how do you plan to hope that of the small percentage of mages who are allowed to live, there are sufficiently powerful ones available from the existing number to even fight for you in the first place? There's no way to predict that either, and less opportunity if you have a general policy of killing most of them outright anyhow. Tad bit of irony there: the weakest mages, those who can barely manage to light a candle, pose the least threat to the world and would thereby have the greatest reason in their favor to not be culled, are the ones who also would be of the least benefit in serving as a military force when needed.
Finally, how do you plan to account for the fact that instituting this kind of a policy is going to create a system in which even more people are inclined to hide mages within their population, and what mages are allowed to live are going to be that much more poised to rebel. Or the fact that certain groups, i.e. the Grey Wardens, since this new policy will make mages an even more precious commodity, will be pushed into more direct opposition to groups which would enforce it? Or, as was already said, those societies which will recognize that this policy will provide an automatic military advantage to any faction which decides to offer mages an alternative.