I don't see it that way. It's not like Jin and Jin had time to sit down and look at the options... and it wasn't a "romantic" decision by Jin to die with Sun, until the very final moment when the choice was between death for both of them alone or together.

Jin couldn't accept that Sun, the love of his life, was doomed. He remained behind to try to save her, not to die. His courage wasn't cheapened by asking the others to stay behind - he knew the odds weren't good and he insisted to be the only one to try. When Jin accepted that Sun could not be saved, he knew it was also too late for him to try to go away. He would drown trying to leave. Instead of cowardly letting Sun face death alone, he faced death with her. A lesser man might have panicked in the face of death and drowned trying to save his skin at the last minute, but Jim remained true to his love and his promise to Sun.

I was expecting Sun to bring up Ji Yeon and argue that Jin had to leave for her sake, but she didn't. She was panicked and facing her death, it's not like she was in a situation to quietly contemplate her daughter's future. It probably wouldn't have fooled Jin either: Ji Yeon was safe, it's Jin's life Sun would have been trying to save using their daughter to convince him. That would have given a very sour twist to the scene: Jin would have looked bad to let Sun die alone, or he would have looked bad by choosing his wife over his daughter. Better leave Ji Yeon out of it, as the writers made sure to explain earlier she was not in any danger or bad situation.

Ji Yeon isn't in any terrible situation. She's not with Sun's gangster dad (now bankrupt), she's being raised by her very wealthy grandmother. Not ideal, but would a Jin forever plagued with having left Sun die alone, forever asking himself whether he really did it for Ji Yeon's sake or because he was too much of a coward to try to save Sun, forever asking himself if he could have saved her would have been such a good father?