It is a very constant fear for all mages. I don't know what DA you played but it seems to be a lot different.
One not in your fantasy land, presumably.
Any mage that's been lucky enough to be educated about their situation are very much aware of possession. That when they dream is the most dangerous time.
It's only a statistically significant danger in a thin Veil area.
You have been shown and told repeatedly its not about how thin the veil is. YOU Xilzhra choose to ignore that and bury your head in the sand. Redcliff is not "thin-veiled" and all the excuses you and the rest don't mean ****. THat happened. SHaled village that happened. Meridiths sister that happened.
Redcliffe was not thin-Veiled, true; that was deliberate demon summoning, which you'll find I mentioned in the post you quoted. As for Meredith's sister, you'll find that the codex states that she was a native of Kirkwall, presumably a village outside the city walls proper; however, certain other codex entries indicate that Kirkwall's miasma slipped out past the walls themselves, and one of the possible sources is Corypheus in the Vimmark Mountains. That whole region is tainted, not just the city proper.
And the demon in Honnleath had also been deliberately summoned... not to mention that there were no abominations in the village.