It's not a matter of bad or good governance but of autonomy.
Being ruled by one's own people should be a basic right of any group, do you deny this?
Even if you do, the mages seemingly don't because they went to war so they could rule themselves.
Therefore, the question is not whether mages make good or bad rules but whether they, by virtue of being powerful, will always end up dominating society if not kept in check as powerful creatures are wanton to do.
The fact remains that every society where mages aren't shackled is ruled by mages. Tevinter, the Dalish, Rivain, the Chasind.
But it is, undeniably, extremely dangerous. And unless there is some equally extreme reason as to why it is necessary, spirit possession means the Seers are placing the people at risk for no reason.
If there is no possession, there is no threat.
Heck, Seekers can't be possessed. Period.
1. The apparent difference between you and me is that I don't differentiate mages by virtue of them being mages. They are people, same as anyone else, born with more power and thus are subject to more accountability to their self-control and need for training, but a human mage is still a human, an elven mage is still elven, and even then they may grow up in a culture where neither their magic or their race matters. If they are born and grow up in the Qun, they are Qunari regardless of their race because that is their belief, and a Circle Mage would be very different from a mage who is trained to be an Avaar Shaman or a Rivaini Seer, regardless of race based entirely on the culture they grew up in.
I don't deny any group should have the right to rule themselves, but mages have long since lost that right based entirely on the disposition of the Knight-Commander in charge of each individual circle.
But if we go by the Circles, the mages were promised a level of autonomy when the Nevarran Accord was first signed. They were promised a place where they could freely practice magic and be governed by a council of their own enchanters because the alternative was being glorified janitors for the Chantry or being killed in an exalted march because they peacefully refused to light candles. The Right of Tranquility was meant to be used as a last resort for mages too weak to handle demons and nothing else, and there was no Right of Annulment. Fast forward to Thedas pre-Asunder and you have a system that is far different in all but technicality in form, where mages don't have autonomy because tranquility is used as a bludgeoning instrument of punishment, Annulment used without justification in Kirkwall and Rivain.
As for the war, the mages as a whole didn't go to war. A very small council after the college was attacked by Lambert before any vote of any kind was made, and even then it won by only one vote. Most mages wanted nothing to do with it. And it wasn't even a vote for war, it was a vote to separate from the Chantry because its templars had abandoned their sworn role and duty in guarding mages as well as mundanes. It was the templars who separated from the chantry so they could wipe out the mages or force them back into circles, while the chantry's position was to let the mages be, so in that point of view it was the templars and the seekers, not the mages, who wanted the war.
2. You have no proof whatsoever that the mages who aren't controlled and leashed to a circle are in control everywhere. The Dalish have mages as guides and leaders, but even they are subject to the will of the hahren, the elders of their clans. The Dalish Warden's parents went behind their clans back to bond and one of the parents was a Keeper. We have no evidence on the Chasind whatosever so you have no ground to make the claim that they are ruled by mages. In fact, there's evidence to the contrary in Mark of the Assassin with that one bodyguard of Prosper's who is a Chasind who breaks the minds of mages, and Rivain has mages as spiritual guides and honored advisors, in positions of influence but don't seem to be rulers at all. Your claim that free mages will rule is severly lacking in actual evidence beyond your own speculation laid out as fact.
And on your topic of spirit possession of the seers, if you want to make the case that it is irresponsible and dangerous, you'll need to come up with specific examples in the lore or history of thedas that shows the traditions and the very nature of the magic they practice is such that it is what you claim, whereas, as I pointed out repeatedly in debates, all the existing lore shows the exact opposite because they seem to have far better results in their system than the Circle system, and a better standard of living for its people than Tevinter, which shows they have a system in place that works and it doesn't need the Chantry or the Circles in order to function.
As for the Seekers, they can't be possessed, because they are already touched by a Fade Spirit. And that doesn't eliminate the danger of willingly working with demons like Lucius and the Envy Demon, alongside all his top lieutenants and the entirety of the the templar leadership.
Also, Cassandra's apprentice apparently had a demon in him, although it wasn't controlling him, if I remember that bit of dialogue correctly. Immune to mind control and possession, but most certainly not immune to corruption. Apparently that Book of Secrets we get from Lucius tells that the Seekers had been corrupt for a great period of time. Cassandra briefly mentions that the Seekers deliberately kept information from the Divine, the one person they answer to, in order to increase their own power and influence alongside them hiding the cure for tranquility.