You are right. They both had no individual good lifes to go back to in the first place. Dohyuns life was fucked up as well. Being an asshole, not caring about others one bit. The only one he ever cared about was Sungho.
My real life morals surely have nothing to do with my moral concepts in fictional storys. Not serparating those two would just be super boring.
Those last few lines were so important to the story line. Yet I still see people saying how they hate the ending because he shouldn't forget what happened. They aren't forgetting their past. They are just moving on cause they sense they can make the future work.
It's like the author said, the ending doesn't necessary need to satisfy everyone other than the Sungho and Dohyun. That's the point of fiction really. It doesn't need to make sense.
And for all it's worth the lines in the last few panels:
"We can't go back, We can't undo things.
All I see is you standing in front of me.
There's no need to back".......really says it all. It doesn't matter as long as they are happy with each other.
A true masterpiece.