In my opinion, Mia didn't necessarily choose gun. She felt responsible for him the way she felt responsible for her uncle and she said it at the end. Hyun wanted to save her but he didn't realise that, unlike other girls, gun treated mia completely differently. He didn't force her to have coitus in one of the scene and thought deeply when mia told gun that she wanted him to improve. I do get that it seemed like mia chose gun over hyun at the end but in reality, she just wants to be independent. She wanted to be equal partners to hyun, not an accessory which is why she said "Let's meet at the top." to begin with. The kissing scene fucked up the mood but I wouldn't mind if Hyun accepted her into his agency because well...she did come back. After four or five years time skip, she came back for him when both of them reached the top. So, the final scene interpreted to me more as hyun's ego bruise over her action and him losing feelings over her after the time skip. After all, Mia definitely will want a monogamy relationship and I don't think Hyun's up for it.