Huh? Did you never watch Star Wars? How did you not know everyone would die?
Star Wars claims that the events of Rogue One were "their first victory against the evil Galactic Empire". Generally fights where you lose 95% of your solders, and the enemy escapes unscathed, are not considered "victories".
So you never watched Star Wars either, or what? They all had to die. It wouldn't have made any sense otherwise.