So like Subin and Hyung would be a more sustainable one even tho he gives no shit about Hyung, uses him than with the guy he is actually in love with and wants to be with? Sure, and Hyung would be happy to hear he is his toy for comfort and shoulder to cry on and that Subin won't ever love anyone as much as Hyunwoo and this wouldn't possible cause frictions btw them in the future if they stayed together. Honestly .... Hyung is better off alone than with Subin.
I think Subin would become a better person through being with Hyung and that he does have feelings for him. I guess you don't understand how hard it can be to let somebody else into your heart when you've been in love with somebody that has treated you like shit and played with your heart for years. I don't think you're reading the amount of depth that is actually present in each of the characters and their emotions here because your comment displays an oversimplification of the plot as well as the characters narratives.
I think theres one thing here nobody seems to be addressing. Hyunwoo and Subins relationship would never be sustainable. They aren't at all compatible and if they got together there is too much that would cause them to break up. They'd be those on again off again couples. They wouldn't make each other better people. Hung and Subin is a much more compatible and sustainable relationship. I do not think Subin would end up happy with Hyunwoo at all. They already have too many problems when together and Hyunwoo has so much to work through before he can navigate a relationship properly