The problem isn't that there is a canonical path, it's that BioWare wants to have choice plus a canonical path that acknowledges every major choice you make which limits what kind of major choices you can offer in the game.
This isn't true. The plot progresses no matter what choices you made. There is no practical limitation. The Blight always ends no matter who dies. The Mages and Templars have a schism no matter what Hawke does etc..
Plus not every choice is even kept around. Killed Leliana? Sorry, no you didn't. Killed the Rachni? Guess who's back! Made Anderson the counselor? Not anymore. I find the whole "consistency" argument a bit silly given that they keep rendering them irrelevant like this.
First of all, the overarching plot isn't static. It can be reinterpreted and rewritten over time for various reasons ranging from writer turnover to realizing that a previous idea sucked. Sometimes a previously minor character can become more important. Some completely disappear. The plot isn't static, but it is still coherent.
Second of all, the consistency remains no matter what is "retconned".
You didn't kill Leliana. You thought she was dead. You left her for dead. The decapitation sequence is a random gameplay element.
The new Rachni Queen is a clone and has an entirely different personality with entirely different consequences should you interact with her.
Anderson resigns because he couldn't handle it anymore. In retrospect, this was rather obvious.
In KotoR I could usurp the big bad and make a Sith Empire that rules the galaxy with me on the throne, which is something that I can't do in Inquisition. I have to be the light side Jedi that saves everybody and becomes the hero of the people because that's the intended canonical save import state.
The Kotor games are technically stand-alone games, as each game has its own independent sequence of events in the established Star Wars universe. The choices you make are entirely removed.
This makes it more similar to TES games than DA and ME.
Now I agree that DA doesn't let you be psychotically evil but I fail to see how it doesn't let you establish your own "sith order". The choices you make throughout the game as well as the choices you make through the War table for the Inquisition are far more tangible than en illusory evil empire in Kotor could ever be. My Cadash dwarf, through his choices, essentially turned the Inquisition into a money-making puppeteer that served his own purposes. Naturally, Corypheus was in my way.
I don't consider the divine thing a failure state, as failure states result in not being able to continue. You can still keep going with Vivienne as the divine, the game just did something kind of weird in assigning her to the role.
How is it weird? In retrospect it's entirely obvious why the choices make sense in supporting one divine over another.
Putting Gaspard on the throne is obviously going to work against Vivienne (political) while helping Cassandra (militaristic). They're not vague either. Cassandra and Vivienne make it clear who they support if you don't care for lore to begin with.
I agree that it would be cool to have a choice to side with Corypheus but it would only work if you lose in short order. This sort of failure already happens if you're a stubborn character who refuses to play along with the Game in the Winter Palace. If you're asking for ACTUAL significant divergence on a path that in no shape or form contributes to the overall setting then you're playing the wrong game. There is way too much story to tell in very little time in order to devote any significant number of resources (and they are very significant) to a non-critical path that won't transfer to the next chapter in the story. This is, again, best left to stand-alone or sandbox games.
Regarding my definition of a potential failure state, I have to ask you one question. Assuming you aren't meta-gaming, are you comfortable importing the DA state with the Divine that you don't agree with into the next game? From empirical evidence, most people would say no. This makes it a failure state even if it's not official. 