If your character is in the middle of progression story and suddenly the other player decided to call quit in the middle progression, the part of that plot because dead as a doornail. It doesn't matter how friendly and how memorable the relations between two characters were. If the plot gets halt in the middle, you can't just leave it as open plot for the rest of that character's life. It's unfair and it's also probably not what you personally wanted from the start. Which is why retconning can be a very good essential thing to players to help them to alter the plans or alter story if they have to.
