Spirits were free to roam in the mortal world in the past and they weren't the issue so clearly a solution that has been forgotten exist one which is far more effective than those prisons called circles.
Each person among the elves had a mortal and a spirit shape so they were immune to possession or at least i never heard about ancient elf abominations.
In the Dragon Age Instead to learn techniques to be immunized against possessions and repel demons they adopt improbable solutions.
There is technique that prevents demonic possession it is called Rite of Tranquility and works perfectly fine .Aside from matter that it was completely different world, and was ruled by self-destructive mages that if Solas was correct would destroy it .
There is a degree of poetic justice in that someone like TKS would see the end of their lives and the society they so dogmatically sought to control and preserve because they refused to acknowledge magic for the natural phenomena it is. Stagnant and incapable of progress, it's nigh certain the Qunari or Blight would overcome Thedas were it to share that mentality.
Ego and lust for power are evidently quite present on both sides, regardless of whether one sympathizes with the mages or mundanes.
And how exactly i would die in first place? Blight wouldn't even exist without mages or if they were controlled properly, and Thedas without constant disasters caused by mages would be much better suited to fight qunari , not to mention technology among humans would progress as mages wouldn't prevent it.
They are tiny minority that threatens its existence.