Via Ew.com:

Rogue One: A Star Wars Story filmmakers have said they always intended to kill off the entire Rebel team during their heist of the Death Star plans on the tropical world of Scarif. But in the very earliest script – before getting the go-ahead for that sacrificial ending – they came up with an escape plan.

“The original instinct was that they should all die,” screenwriter Gary Whitta tells EW. “It’s worth it. If you’re going to give your life for anything, give your life for this, to destroy a weapon that going to kill you all anyway. That’s what we always wanted to do. But we never explored it because we were afraid that Disney might not let us do it, that Disney might think it’s too dark for a Star Wars movie or for their brand.”

So in the original treatment by John Knoll, and in the first script by Whitta, a few of the key heroes survived the final battle. But the creative team still wanted their noble sacrifice.

“You have the darkness that’s in the undercurrent of the story at that point, but you still have the rightness of why they’re doing it,” says director Gareth Edwards. “It doesn’t feel depressing. It feels like you want them to succeed at any cost. It’s a sport where the clock is ticking, and they need to just dive across the finish line. You do whatever you need to do to get there. It’s a gauntlet that they’re handing to Princess Leia. You get that moment where the crowd feels like it can cheer at the end.”

So that argument had to be made to the Lucasfilm brass: the heroes would succeed in stealing the plans, but they should pay the ultimate cost for that victory.

“We were still scratching the itch that they all needed to die. Chris Weitz [who wrote another draft] thought we were right,” Whitta says. “They finally went off and fought for it. We told them, we feel they all need to die, and [Lucasfilm president Kathleen Kennedy] and everyone else said to go for it. We got the ending that we wanted.”