Cassian Andor was always someone who got to walk away. He got away from the fallout of the mining disaster on Kenari. He deserted from Mimban. He walked into an Imperial Naval facility and walked out with a star path unit. Even in a place like Ferrix, we see that he has walked away from his debts, from the labor yards, from a roster full of women. He evades authorities on Morlana One, Aldhani, and Ferrix—twice. He breaks himself out of prison! Whenever he is boxed in, whenever his agency is tread upon, his first instinct is to commit himself to the way out.
Alongside this uncanny ability to climb out of every pit he is thrown into is Cassian's tremendous capacity to show up for the people that he loves. He may leave, but if you are family, he will make every effort to come find you. This is not a contradiction in his characterization—at least not a kind of contradiction that isn't also inside every actual human being—these are two very real pieces of himself that swing between being in conflict with one another, and coming together to facilitate tremendous acts of courage and feats of survival. Cassian is so good at what he does because he is driven by this particular kind of selfishness and selflessness all at once.
So when he tells Bix that he is done, that this is it, he has given enough to the Rebellion, that's consistent with the man we know him to be until this point in his life. He's simply taking another step forward in the cycle that he has already been living in, and it is this very cycle that Bix comments on when she makes the decision to leave. Bix has come to understand what Luthen already knew when he gave his speech to Lonni, what Saw has known since his trial by fire on Onderon, what Lt. Gorn knew when he turned his back on the Empire: rebellions may be built on hope, but they are won through an immeasurable amount of sacrifice.
We've seen the sacrifices others have made willingly. the Imperial maintenance worker abandoning her own future. The Ghorman bellhop bombing the hotel he could never bring himself to leave. Mon Mothma abandoning her family. Kino Loy climbing though he knows he cannot swim. Cinta crashing a car with the express purpose to kill the passenger seated behind her, despite having no way to guarantee her own survival. And now Bix, giving up not only Cassian himself but the security and stability of Yavin, the family she has found with Willmon, and the future she desires most.
Cassian has been transformed by each of these people, but has yet become the person he needs to be to fulfill what he is only now starting to realize is his destiny. That destiny is as much written by the natural course of war and rebellion as it is preordained by the force, and under pressure from both Luthen and Bix, he is arriving at both of these realizations at the same time. As for Bix's sacrifice, she knows the only way he becomes the needs to be is by breaking his cycle, and the only way for the cycle to be broken is by forcing Cassian to break it himself. Put him in a box, and make him find the one way out.
This all really worked for me. I do wonder if there is some kind of total inversion of our understanding of Rogue One Cassian in the works. I know they say we will see the movie in a new light, but how far does that go? Will Cassian's heroism still seem genuine, or will it feel like he's finessing? Will he be a scorned lover reaching for something he knows he can't get back? Will he seem like a charismatic madman, driven by a singular purpose to make all that sacrifice worth it?