Think back to the pain and horror we felt when Luke arrived at his homestead, to find his murdered Aunt and Uncle. It was as if we were there with him in the speeder, getting closer and closer, as we watched the grisly scene come into focus. Forty-one years later, and I get goosebumps and a sick feeling in my stomach each time I watch it, or even just hear the music from the scene.
Well, we'd actually spent about twenty minutes getting to know Luke and his home life and relationship with his guardians and thwarted dreams and such. Apart from a couple intercuts to the Death Star, we don't even leave Tatooine until almost an hour into the movie. See my persistent rant about overly-compressed story from even Empire on, steadily getting more drastic.
I'm not suggesting R1 should've devoted as much time to Jyn and her parents as George did with Luke interacting with his loving, if stern, guardians. But *something* of Galen and his daughter before his kidnapping would've gone a long way to make me care about her.
This is actually why I don't see her as the main character. Any attempt I made to give her more depth skewed things too far away from the story being told. She and Galen parted so many years before the current events, a third of the movie would have needed to be a flashback to establish the same dimensionality. Meanwhile, by co-opting the Cassian & K-2SO one-shot comic it makes a good first act for a Cassian-centric story, with Jyn working fine in a role with about as much development as Leia in ANH.
His arc also better follows the Hero's Journey scaffold. Empire cost him his birth family, found a surrogate in the Rebellion... His "Call to Adventure" is the niggling feeling he's being tasked to do things that make him no better than those Imperials who killed his parents. His "Call Refused" is his denial of that sneaking feeling and continuing to do his job. His crisis point comes when he chooses to save Jyn's dad rather than execute him as ordered. After that I'd be fine with him survivng or not, as long as either way it's organic to the story and authentic to who he's become.