Wait, so you're saying that in a fantasy universe with two or three different types of mind control and 3 to 4 different types of possession by supernatural entities, that a supernatural entity being able to command you at a whim after you made a bargain to take in some of its power is a non-real made-up thing?
In a fictional universe, you have to examine something like that by comparing it to how magic has been established as working within that fictional universe. And magic in Thedas works by taking a part of the Fade, placing it over some part of the physical world, mixing it up, and then controlling the Fade to bring about a physical alteration to the mixed Fade-physical world.
A geas which has no physical existance, no physical property at all, which is a complete nonexistance is not something which can be brought about in the dragon age universe because it cannot be brought about by making an alteration to the physical world as a geas has no existance at all. In fact, a geas has no actual mechanism that makes it work. It is just an artistic tool that storytellers in Ireland and other parts of northern Europe employed to move the story forward. Thus the geas was consistent with the fictional world of those ninth century storytellers as the geas was how magic worked in their fictional worlds, but a geas is completely inconsistent with how magic works in the DA universe.
And there was no point in even introducing something that is so inconsistent with the DA universe when there are already plenty of consistent methods of accomplishing the exact same outcome.
I know what some of you are probably thinking, that the Geas was could be explained as a physical alteration of the Inquisitor's brain. However this is wrong for at least 2 reasons:
1) Magic requires a caster to transfigure the Fade over the physical world and then bring about the change to the physical world. There was no caster.
2) It is impossible for anyone or anything to be able to use magic to alter someone's brain. No one in Thedas has any clue how the brain works, no idea about the chemical interactions, the cells, the molecules, nothing. In the real world we know what the brain looks like, what the cells look like, what the molecules and atoms look like. We have a general idea about how each of these things bring about memory, emotions, etc. And even we can't alter someone's brain in this fashion.
The best that a DA mage could do is use blood magic to turn someone's body into a puppet for so long as the blood mage is able to control that person. But once the blood mage runs out of whatever they were using to continue altering the physical reality and thus moving the other person's body, their control would end.
"Brainwashing" in Thedas is achieved by inserting a demon/spirit into someone and having the demon/spirit assert control over the person's body/mind/soul by replacing that person self with the demon's self. But that didn't happen either.