You base this reasoning off of special snowflakes though.
When in truth we do not know the commonality of possession attempts or their success rates, what is known however is that Mages face demonic threats on a fairly regular basis. So the argument that possession should be treated as a minor threat to all mages when both the lore and game play provide entirely different stories doesn't hold water.
"Oh but the Mage PC resisted demonic temptation and possession!"
Them not implementing a game mechanic for it doesn't make possession impossible.
but thats's because you can't easily implement a game play mechanic for it.
a circle mage is better trained to resist possession then anyone else.
mages are a threat of being taken over for 3 reasons.
1) they ATTRACT DEMONS, their more easily found in the fade then a normal human
2) in the fade their in the demons domain, its here they can be "trapped"
3) Demons can whisper to mages whenever they "tap" into the fade, Especially when using blood magic.
the increased risk is not a mage walking down the street and suddenly "OH NO DEMON ATTACK"
its the fact that they can be driven nut's or tempted to accept offers from the actual demonic voices they hear.
Thats the whole point of Uldred when you meet him in the harrowing chamber, the demons can't just take over the circle mages, uldred Summoned the pride demon into thedas and not just any pride demon the same all mighty pride demon that had been trapped in a certain section of the fade as a test for the harrowing which then bull rushed him, as it could literally anyone even a templar.
the rest Uldred was torturing until they gave in and allowed the demon to take hold, the demon itself could not take over unless the mage gave up, which the demons couldn't do it was Uldred.
the only other possessed mages we come across are dead arcane horrors or dumb arse's who accepted deals from desire demons.
It would be lore breaking if a demon tried to just physically take over a mage walking around thedas and stupid if it was a quick time event. but If they were able to somehow add the whispering influence of demons into the gameplay, even if it were just for blood mages THAT WOULD BE COOL.
Imagine a fade spirit weighing in on your choices, urging you to take a certain route, or telling you not to trust your party members, even possibly giving you advice that saves you in an attempt to earn your trust.
thats how mages turn through their connection with the fade, physical take over is no different to anyone else. So a silly quick time event makes no sense. possession makes more sense of being a risk overtime, hallucinating etc. being DRIVEN TO BREAKING POINT rather then just BLAM YOUR AN ABOMINATION!