I think the notion that the Templars can just reform and switch into "guardians" is far fetched.
No more far-fetched than a mage on the Sunburst Throne or especially softened Leliana's massive effortless reforms, IMO.
Whatever they want to do, given their past conduct how can you expect the mages to trust them? And without that trust, what option will the Templars have except to be jailers?
First, many mages never wanted the war. Not all mages are slavering templar-haters, just as not all templars are slavering mage-haters.
Second, many of the extremists on either side are dead -- like the ones we deal with in the Hinterlands. The templars who survive Therinfal and join up with us are those who hesitated and fought back instead of following orders blindly, and got an undeniable demonstration of how far the order had fallen.
Third, Cullen mentions the templars and mages of the Inquisition starting to bond with each other. Now that's admittedly "just" our own people, but that should still be a not inconsiderable number. And they can carry those experiences with them even if they leave us later.
Fourth, our templars do reform and become into guardians. The war table missions show them doing exactly that. They protect innocent mages from angry crowds, investigate accusations to see if there's merit to them instead of just killing every mage on principle, and offer amnesty to mages willing to abandon the blood mage cult they got roped into. And the leader who reports back to the Inquisitor expresses relief that they're able to serve this way after Therinfal.
Fifth, not even Vivienne wants to put things back exactly how they used to be. I definitely agree that would not go over well and would just lead to the same sh*t happening again sooner or later. Thankfully, mages will have more freedom than before, no matter who becomes Divine. With Cassandra or Leliana on the throne, there won't even be "jailers" anymore.
And unlike the mages, who can't escape their own mistakes so easily, the Templars can stop being Templars by the stroke of a pen.
Two words: lyrium addiction. Cass is right about the fact that the templars have had even fewer ways to air their pain than mages did.