1) You can learn blood magic from books or other blood mages. BW keeps changing their mind on how to learn Blood magic, so the explanation that it can only be learned from demons doesn't hold any water at this point.
2) Because most of the blood mages are desperate, crazy or stupid.
And it doesn't always lead to bad stuff happening, especially with people who actually know what they're doing. Jowan uses it to save a life (at the willing cost of another); Avernus used it for much needed research into the Blight; Merrill restores an eluvian; Malcolm Hawke sealed away demons and Corypheus; Morrigan uses the Dark Ritual to save the Warden's life; and don't the Chantry use Blood Magic to track Rogue mages with phylacteries?
3) Noted though you can also summon spirits with Necromancy. More ever, Rift Magic draws its power directly from the Veil itself for some powerful effects. Wouldn't that logically weaken the veil if the mage decides to draw too much from a particular area? Or if they misjudge and use their magic where the veil is naturally weaker? Not to mention that Rift Magic can already outright kill its user?
4) You can use similar or worst effects with other schools of magic especially Entropy
5) This one makes no sense. How are you more prone to possession if your connection to the Fade weakens from Blood Magic?
Okay? One magister says that blood magic isn't worth it. They're opinion shouldn't be treated as absolute, simply their point of view. Even Solas notes that "blood magic is no worse than any other magic if properly used." He says this while acknowledging its risks and dangers. Dorian is uncomfortable with blood magic, but agrees that its okay as long as you only use your blood or that of a willing participant. So there are respectable mages who both refute and acknowledge blood magic in-game.
1) I never said that it can only be learned through demons, only that it is traditionally (read: commonly) learned from demons. There are exceptions to the rule, but they are extremely rare, so much so that a templar or anybody is going to stop and think, "hm, maybe they learned their magic from a book or something."
Which, by the way, is extremely stupid, because if all magic can be learned straight from books, then what is even the point of having a mentor?
2) Jowan's method involves killing someone to save another, as opposed to using lyrium and some mages. Yeah, sounds good to me.
Avernus' research and use of blood magic results in the tower being possessed by demons (and consequently, Demon Sophia), and also the Veil being torn open and only TEMPORARILY sealed. As soon as he kicks the bucket, that seal goes away, and it's only because of YOUR help that it gets fixed. Yeah, sounds good to me.
Merrill's eluvian would have been used by a demon to escape it's prison and possess her, except the keeper intervened, sealed it within herself, and then died as an abomination, all in order to save Merrill from that fate. Mm. Yeah, that sounds good to me.
I haven't played Legacy yet, so I can't speak for Malcolm.
Morrigan saves the Grey Warden's life, but also damns her son to being a schizophrenic and almost loses him to Flemmeth. As opposed to being a normal boy. Sounds good to me.
Phylacteries are responsible for the deaths of multiple mages who just want to live free lives, which is the entire reason the Mage/Templar War kicked off. Mm. Yeah, sounds good to me. Just because the Chantry does it, doesn't mean it's good or for good.
3) Not much is known about rift magic, because it's a completely new field that came about as a result of the Breach and all it's wonderful shenanigans. You can argue about that all you want, but there's no supporting evidence for it or against it. Similarly, I can't seem to find a whole lot of information about necromancy, other than it's creepy, weird, but uses wisps/minor spirits to reanimate corpses, similar to how a spirit warrior would use spirits.
4) Entropy doesn't take hold of their WILL, and just because effects are 'similar' or 'worse' doesn't mean that the processes and effects are exactly 1:1. I can burn someone's hand, or they can touch a hot pot, but that doesn't mean that both should be evaluated (morally) the same just because the results were the same.
5) You're putting words in my mouth, here.
I never said that you were more prone to possession, only implied that your overall magical ability is weakened (due to your ability to access the Fade being weakened). You'll eventually stop being able to dream, even, so who knows what happens to your soul when you're dead?
Also, Solas is someone who damned his own people in his attempt to save them from slavery, and is also starting a race war with his endgame being that there will be no more Veil, period, and all of humanity being destroyed in the process. Respectable people my foot, people in-universe who argue in favor of Blood Magic all seem to be entirely short-sighted and not thinking about anything thoroughly.