I myself am starting to doubt if Mafuyu and Ue are even meant to be together. All of these emotion are no doubt absolutely terifying realistic and I LOVE YUKI but it is getting a bit frustrating for me. Are they even going to progress or what?
When the manga, anime become famous, making more money, we would see something like this involved. Doesn't matter how much Mafuyu and Yuki love each other, Yuuki will never come back to feel and do anything. It's Ritsu who's there to feel happiness and pain, love and share everything with Mafuyu. The development between Mafuyu and Ritsu is too slow or zero. Saying he likes Ritsu now, but I don't see Mafuyu's doing or start takting Ritsu into his life at all. Ritsu's sister's words are back to my head, Ritsu will get hurt from this, really? Don't you think it's time to move forward Natsuki Chan?
Maybe it's slow for you, but that's one thing i personally enjoy about this manga. As i've already said, you love people differently, and after everything Mafuyu's been through, i don't blame him for taking it slow and being cautious.
Though i do want to see more of the feelings he has for Uenoyama, i want to see him accept Yuki's death and try his best to move on. But the thing is, that you can't force it.
Yes, Yuki won't come back and the person who is there right now is Uenoyama, but as i've said, that doesn't matter.
You love someone and then you lose them, only to get feelings for someone new even though you haven't processed your feelings of loss yet, you haven't worked through the trauma and things bottled up inside you, so it's no wonder it's difficult for him right now.
I suspect that's what Natsuki is trying to get across, though it doesn't seem to be to everyone's liking. They're trying to make it realistic and with realism, there are going to be people who won't like it, so i can't exactly blame you for thinking like this either.
Take the story and development between the characters with a pinch of salt, basically.
I feel like the way Mafuyu shows his emotions and how difficult it is for him to accept Yuki's death...is so realistic in a way that makes me think.
Like, people are talking about how much the manga focuses on Yuki. So much to the point that it apparently makes it seem like Mafuyu doesn't love Uenoyama.
Which we all pretty much know isn't true. In the first place, you show love differently. If i'm not wrong, the way he treated Yuki is about the same as how he is with Uenoyama, but added with reasonable emotional difficulties.
You don't just stop loving someone just because they're gone and you have someone else in your life, though it looks like that's what several of you want to happen.
I can only imagine how painful it is to lose a loved one, and to the person who's closest to you no less. Yeah he has a new significant other but they haven't known each other for nearly as long as Mafuyu and Yuki did. And come to think of it, losing someone by suicide, after the kind of fight they had...of course that's going to affect you.
TL;DR
I personally think the main focus of this manga is losing a loved one, and the aftermath of trying to cope with the loss.
Yuki and his relationship with Mafuyu is supposed to be brought up a lot, because it's a big part of Mafuyu and his character.
So if i see anyone complaining about this very thing, genuinely, please stop.