1 never really happens in this game. 2... there are still Fade rifts around, and it's easy enough to see that no one really wants to destroy the entire Inquisition out of spite.
I was not talking about the game as it is, but about the game where blood magic exists and is properly acknowledged. Cheap plastic imitation of blood magic that only exists in the combat subgame but not in the story subgame is just a waste of developer time.
By the end of the game, the only two ways a blood mage Inquisitor could NOT remain in power is if (1) an Exalted March or something of that magnitude was declared on the Inquisition. Since the Divine is from the Inquisition and you've helped the major players of Southern Thedas, this is unlikely. (2) a small-scale assassination attempt. This scenario does not exclusively apply to a blood mage Inquisitor, though; ANY Inquisitor could be subject to an assassination attempt.
This only applies in the scenario, where the Inquisitor plays their cards right. It's also possible to elect a new Divine who actively hates the Inquisitor. The new ruler of Orlais may not be a friend or an ally either. Many of the companions/chief lieutenants could also have negative approval. If the Inquisitor is a blood mage, the Inquisition itself may be their biggest enemy by the end of the game.
Many things would be easier, if Dragon Age games were just individual games with no continuity between them. The writers wouldn't have to deal with a situation, where player choices need to be acknowledged within a game, while still making sure that they have no real consequences after the game. Then we could have a blood mage Inquisitor who 1) ends up being assassinated by their companions after Corypheus has been dealt with; 2) survives the assassination attempt and flees, leaving the Inquisition without any high- or mid-level leaders; or 3) survives the assassination attempt, solidifies their hold of the Inquisition, and becomes the next major villain in the South.
Neither is blood magic. Fanbois here will defend any decision done in the game just because. That does´t mean it is not possible. If you pay me enough I will write you a story of Inquisition with blood magic. Simple, when there is a will there is a way.
But will it be the same story with the same consequences as the story of DA:I?
In a story-driven game, the story comes first. Player options are determined by what the developers can fit within the boundaries of that story without significantly altering it. When developing DA:I, BioWare came up with a story that didn't have room for blood mages. In DA:O and DA2, blood mages could have fit in, but BioWare still decided against them due to the lack of resources or the will to do it. In the end, those games had just ordinary mages with a few "blood magic" spells.
That is just bull. The qun is THE monster which tried to engulf whole of Thedas. Turn people into mindless tools and destroy any free will. In another words mental blight. Anyone opposing will be destroyed, be it either physical death or mental destruction. You can´t whitewash such monstrosity and then turn around and say one blood mage is impossible. Sorry, you are just shooting your credibility out of the window.
We've obviously played different Dragon Age games. In the games I've played, only a few fanatics have views like that.
(As a side note, the idea that you can either convert or die has been central to both Christianity and Islam for a large part of their history. Religious freedom is a relatively rare phenomenon.)