I don’t get the argument that the characters didn’t get enough background, such as not knowing how Baze and Chrrut met. Who cares how they met? You never find out the backstory of Jules and Vincent in Pulp Fiction, and those characters are still terrific. Hell, you never find out the backstory for Han and Chewy, and those characters are loved by all.
Someone gets it.
If one wants character arc, they’ll have to reduce the cast or make a series of films with the same characters. But that wasnt the intent of this film. Nor is it an inherent pre-requisite for all cinema.
But from a certain point of view, one could say the REbellion as a whole has been nothing but a flat character in all the other Star Wars films combined. Rogue One did not develop any single character to any degree but they are still likable characters. They are still relatable. And the Rebellion now has some depth. Previously they were nothing more than Star Trek’s equivalent of the disposable crew member.