I mean the more I think about the ending the more I like it. It not only fits very well to the over all tone of the story, but it makes sense. Now granted I understand why people are feeling negative towards it. Despite upholding a more realistic, and serious take its still a manga. So people may be hoping that the ending will be a bit more detached to allow for a more rewarding conclusion. Now to get back on topic I think the conclusion fits very well with the over all complexity of conflict that both of them had. Koto is allowed a good ending, because her conflict despite how serious it was was centered solely around her. It really wasn't about her mom, now granted her mom was used as the vehicle to further her despair, but her inner conflict was about her struggling to stand up for herself. Once that was resolved she was relatively fine. Sure its serious situation, and in no way do I wanna discount it, but when compared to Kousuke its not as complex.
The conflict that Kousuke faces is more complex, simply because of the fact that the conclusion isn't visible. I mean the manga as a whole is proof of this. He gains the affection of Koto, but he loses the affection of Miya. Although some of us wanna talk about how ungrateful Miya is, and how she is undeserving of love. She is still his daughter, someone that he loves. So for Kousuke the conflict has evolved. Now granted the type has stayed the same, but the reason behind it is so much greater. Hes in a conflict of trying to get Miya back. I still think that Miya is the only person that will allow for Kousuke to really heal. Sure he loves Koto, and at a time has chosen her over Miya, but shes family.
So I think the author made a good decision by only allowing Koto to resolve her conflict, whereas Kousuke has to face a seemingly adult issue.