Templars are trained and drugged
Since Lyrium does not alter the mind of Templars to make them obedient, then they can't be said to be drugged to follow any ideology.
To protect the people of Thedas from the harmful effects of magic.
While demons can possess mages, this is usually caused by stress, which tends to be caused by fear of what the Chantry and Templars will do to them.
1-This avoids the main fact which is that Templars can strike or not, mages can never choose to not be a gateway for demons.
2-Chantry and Templars are not the sole source of stress or fear in the world. Hunger, debts, bandits, sickness, slavers, unrequited love, the sources of pain in Thedas are literally without end. If you argument is that mages need to leave carefree lives that the people of Thedas don't enjoy in order for them to not harm others, then it is a self defeating one.
There'd be fewer abominations if the Chantry was less draconian, the Templars didn't exist, and mages could train in a less stressful environment.
Despite the fact, as I said above, there are a million ways for a person to suffer from stress.
There is absolutely no reason to believe that fewer Templars would mean fewer Abominations.
And even if it did, it doesn't matter in the sligthest because of simple logic. Let's say with the Circle system, 4 out of 10 mages become Abominations and without it, 2 out of 10.
With the Circle system, those four Abominations are kept away from population centers and are surrounded by specially trained knigths who can respond to the threat within seconds. How many non-mage civillians died from Uldred's insurrection which involved innumerable demons and Abominations? Not one.
Without the Circle, those Abominations ocurr in the outside world. How many civillians died from Meredith's sister or Connor? Hundreds.
Therefore, even if the Circle system produced more Abominations, which has yet to be proven, it doesn't change the fact it is still, by far, the more effective system.
Furthermore, you are only taking into account the possibility of mages becoming Abominations without adressing the very real one of free mages coming to dominate society.