What's the problem with your story being predetermined in a RPG?
Nothing. It's just BGs aren't a great examples of defining your character with custom backgrounds, and I don't think BG1&2 approach works when you go for mainly a coop RPG like Larian does.
If having the same level of character background detail and interactivity with the world as the "origin" characters means only being able to select from a short list of such backgrounds pre-generated by Larian, I'd be okay with that. My issue with the origin characters being forced on us is not so much their backgrounds as it is the mechanical elements of a character (race, gender, class, ability scores, skills, equipment choices, etc.) and the aesthetic elements (the character's physical appearance).
So essencially Origins from Dragon Age, rather then pre-made characters from D:OS2.
On a side note, in D:OS2 you could change character class and their looks to some extend (not the race though). I don't know if the same will be true with BG3. Especially in some cases background seems to be tied the the character. Perhaps at least some range of flexibility (like companions in Deadfire having couple options available) would help. In addition, with multiclassing we still have a pretty wide control over our character even if they start with certain class on lvl1. The weak point of D:OS2 was a lot of freedom but little reactivity or response from the game. Therefore pre-made characters had a bit more meat to them as they had at least some scripted responses.
