I would argue that "mostly" is a overreaction of your part but everyone is entitle to their opinion. Also the ending is not the problem here despise being disappointing, IMHO of course.
Unfortunately that was the most realistic ending for this, ML need years of therapy to overcame his fear of having someone in his life that he can't cut ties with. Maybe one day they will be together or maybe not....
From what I understand from this ending is that Jin Do Joon and Yoon Hyun Woo have the same soul and each one…
There is no soul my friend only timeline....it's the same timeline. The problem here is that they show us the future and the past as the present. Welcome to timeline now expand that to multiverse and go crazy
The problem is that they didn't explain what exactly happened like when ML was in the present time in the beginning…
It's the same timeline my friend. He is Jin Do Joon and Yoon Hyun Woo, its happening at the same time. The problem is that they show us the past and the future as the present. Messing with timeline is confusing af, not every writer is preparer to do so.
I'm flabbergasted