I agree with everything in the OP whole-heartedly.
The origins need to come back. To me, this is not a point of debate, it is a plain, unarguable fact. I honestly cannot add much more to what was already said - the OP especially was thorough and comprehensive - but that will not stop me from trying 
The key word is involvement. I used to be a bit "meh" about DA:O in the past, but like fine wine and classical music, I have come to appreciate it more in my old age. The origins give the Warden context, a place in the world and in the grander scheme of things; he/she is not simply plopped down on top of a pile of debris, but the game says "Hello. This is you. And this is who you are, where you belong and what you connect to." It shows you that yeah, you are the silent fantasy RPG protagonist, so naturally you are the center of this universe, but you are also more than that. I know this is an odd thing to say about someone who, by their very nature, is already the most important person alive, but it creates an environment where things do not simply revolve around you; there are strings, connecting you to events and people happening and existing in the game. The Warden is more than just a main character - by having an origin and pre-existing relationships we get to experience, they become a person.
I usually play Cousland (for some reason, I have this weird thing about non-human protagonists, although I do adore dwarves to death and really appreciate having race options, even if I may not use them myself), and I cannot possibly imagine the main storyline having anywhere near as much impact if I did not get to run around the Cousland castle, interact with family, friends (or awkwardly ask about the well-being of offspring of family friends I most certainly do not have a crush on), and, yes, even Howe, or perhaps especially Howe. Life is good, right? Wrong. In the very next moment, all of that is taken away. From that point on, the motivation of your main character is not only about stopping the blight or dealing with Loghain; this sh*t is personal. That final confrontation with Howe, the outcome - honestly, to me it was more satisfying than resolving the whole Loghain situation later, or even the ending (fire ballistae at Old God dragon, dodge darkspawn, repeat until Old God dragon status = dead). It packed enough emotional punch that I had to run off looking for a mental equivalent of raw steak. Okay, that particular metaphor does not work, but you get my point. I mean, if nothing else, then just the fact that we are still here, years after the fact, whining for more of it has to mean something, right?