The only happing ending I am willing to accept is that one where Seo Joon realises that he deserves better and walks out of there. 

Ji Woo values his own pride more than his relationships. He would rather be the one leaving than be left behind. And this is not a great trait, to be honest. If he ever does something bad, he might not even confess, apologies, or try to fix it. He might just pack his bags and leave. I am not saying he is a bad person, he is probably the kind of person who would think he deserves that loneliness as consequence for some mistake he did.

It took a surprising turn, and for the better.

Good point. I just saw him immature person unable to confront his feelings. Then I remembered there is an age gap. So this is his lesson learned.  At first, I felt like it was really stupid. Now, I feel like it was just growth that had to happen.