First let me say, I didn't have a problem with the 2nd ML/FL, usually the characters provided an opposing view of the main ML/FL relationship. It wasn't that they had more screen time, their screen time was utilized to move their relationship forward, when this didn't occur with the mail ML/GL.
I would have rated this drama higher if, A. The ML/FL relationship wasn't dragged out exhausting. Also, Esther's character didn't develop mentally, it is almost like she digressed after episode 29 or 30. She is supposed to be a professor who experiences a serious hiccup in a relation, but still running around whining, WHY?!?!. Vin character became more open and warm, but this series could have been wrapped up in 25-29 episodes. The OST were fine, not spectacular, but when with the mood of the storyline.

The 2nd ML/FL had a much more marture relationship and probably could have been a separate drama.

Maybe C-dramas will get to the point where characters show up in one drama only for their full stortlines to playout in a standalone drama.

Lastly, this drama didnt require the "interference girl" it served on real purpose to the post.

All in all, it is an enjoyable drama and one of the better I've watched this year.