It’s not clear to me that the characters were actually disposable — at least any more so than in any other pre-ordained narrative. We already knew the ending, but that’s the case for a great many historical event films, comic book films, etc. for which the events are externally sourced. It felt like a war film that canonizes fallen heroes — we all know they die at the end (we’ve seen A New Hope, after all), but their “sacrifice” is all part of the mythology.

So, what, precisely, is the significance of the fact that the main characters aren’t going to stick around for any other film? For me, having the characters die was the only honest way to write this film. Where, exactly, were they supposed to go once the narrative butts up against A New Hope? It seems like there were only two alternatives: (1) try to squeeze pre-existing characters into the main roles of this narrative, or (2) not tell this story at all. (1) would be a disaster. (2) would be fine… but I believe we’d have missed a lot.

For me, Star Wars was always about one of two things — Anakin/Vader (the rise, the fall, the brief redemption, etc. etc.) or “the world” (or both). Rogue One seems to focus firmly on the latter — it’s about filling in narrative cracks, adding small elements that could be picked up in other narrative threads (e.g., regarding the Jedi, regarding rebellion splinter groups, regarding complexity in the empire with traitorous officers, etc.).

Stealing the plans for the Death Star is simultaneously really trivial and, in the context of the overarching Star Wars narrative, pretty important (and the fact that we already knew the ending doesn’t negate that fact). Furthermore, they didn’t simply steal the plans — it was a huge ordeal that ended with the deaths of… a lot of people. They could have simply left it to our collective imaginations, but I believe there’s value in filling out the world that already exists (re-examining it, strengthening the core elements) rather than hyper-extending it, like they did with The Force Awakens (which was a far, far less interesting movie, as far as I’m concerned).

The lack of complex or enduring characters doesn’t make it any less compelling for me. You’re right that the events in Star Wars films are generally relatively silly, at least on the surface. But, by and large, the characters in Star Wars (particularly the main characters) are more silly than the events. The only character in any of the films with any real depth was Anakin/Vader. In my mind, it would have been completely strange to write complex characters — because it was so rare to begin with. Many characters endured for at least a few movies, but that neither makes them complex nor sympathetic — I couldn’t have cared less what happened to Luke or Han or Leia or whomever, except insofar as it related to the overarching narrative. In any case, Jyn had a much more complete backstory than Finn, Leia, and a variety of others from past films.