It turns out that there was a point in the very early drafts of the screenplay that at least some of the Rogue One team made it out alive. Gareth Edwards talked about it on some podcast. But it wasn't a situation where the studio execs were insisting on a more upbeat ending:
I mean, it’s a great Disney tradition isn’t it? For ever single character to die in all their movies. I think there was an early version – the very first version they didn’t [die] in the screenplay. And it was just assumed by us that we couldn’t do that and they’re not gonna let us do that. So we’re trying to figure out how this ends where that doesn’t happen. And then everyone read that, and there was just this feeling of like, “They gotta die right?” And everyone was like, “Yeah, can we?” And we thought we weren’t gonna be allowed to, but Kathy [Kennedy] and everyone at Disney were like, “Yeah, makes sense.” I guess they have to because they’re not in A New Hope. And so from that point on, we had the license and I kept waiting for someone to go, “You know what, can you just film an extra scene where we see Jyn and Cassian, they’re okay, and they’re on another planet and la la la…” And [that] never ever came, and no one gave us that note so we got to do it.