Firstly, mages were oppressed. Some were raped and sexually abused, in multiple Circles.
Do you REALLY think that if mages were allowed to roam free they wouldn't commit rape and sexual assault aggainst the common people? Look at what Denarius did(or was implied to have done) to Fenris. With their powers, the average man or woman has no defense if a mage wants them in such a way. At least in the Circle system the templars, if lead by dutiful Knight-Commander and supported by a strong First Enchanter, there are ways to protect the mages. If mages roam free, however, there is no such protection for the commonfolk.
Under Meredith's command mages were made Tranquil as a punishment, including some who were Harrowed. Cole's whole backstory is about a mage who was left to starve to death in a cell by the templars. After Kirkwall one of the Circles, I think the Rivaini one, was annulled
If a system is run by bad people, it doesn't mean the system itself is bad. One tyrannic Knight-Commander and a few templars that failed in their duty does not mean that the whole system is without merit. That's like saying one bad president/prime minister means the entire government needs to be torn down.
Even some of the day to day working of the Circle is horrific. Mages couldn't have spouses or children.
I agree with you on mages having spouses and children. It's one of the biggest failures of the Circle system, IMO. Denying the chance of family to mages leaves them unconnected to the world. And a person who feels detached from society and other people is a dangerous person to have power.
The Harrowing ritual is just tossing untrained kids to demons and hoping they survive. You don't have to be Anders to notice that mages have legitimate grievances.
Mages undertaking their Harrowing are many things. Nervous, scared, untested and bewildered. But they are not untrained. You don't go through your Harrowing unless your First Enchanter thinks you're ready. You've been trained since your powers manifested on how to control both them and yourself. You've been told, night and day, that demons are dangerous and want nothing more than to trick you into giving them what they want. If a mage is going through their Harrowing, it means they are ready for it. That most mages seem to pass their Harrowing is a testament to their training. That some fail is a testament that the Harrowing is an absolutely necesary trial if a mage is to be trusted.
Secondly, mages integrated into society could have massive benefits for the public. Magical healers are literally centuries ahead of non-magical ones in terms of efficacy.
Considering that healers work and operate in society through the Circle system already, this point is not in favor of abolishment of the Circle system.
In ancient and modern Tevinter magic was and is a major part of architecture and construction, with highways and buildings built to last thanks to magic.
And slaves. Don't forget the slaves.
Thirdly, of course systems can persist for nine centuries without majority support. That's because the societies in question are monarchies/theocracies; they don't have to pay any attention to people who aren't powerful nobles.
The Circle System encompasses all of Southern Thedas. It crosses many cultural and geographic boundaries. It is based on the shared faith of millions. if the people REALLY didn't agree with the Circle system, there would have been some kind of movement to free them before now. Some ruler looking for power would have freed his Cirlce mages. Some peasant mob would have stormed the Chantry and demanded that mages be free.
Instead, it's clear that most people in Southern Thedas agreed with what the Chantry says: Mages are dangerous, and we need some kind of system to keep watch over them. The Circle is it. The commonfolk have good reason to fear what mages can do. The Chantry warns them of this, and for good reason. Southern Thedas may not be a democracy, but there are some things that the vast majority of the commoners, nobles and church could all agree on. The necessity of the Circle was one of them.