It's more of her attempt at giving her fans closer but also a way to continue and connect the stories she told before in Fuyu no Semi and vol. 7-11 of this series with Iwaki and Katou.

Below is a link to the full English translation of the AFTER story. Nitta talked about how she played with the themes of reincarnation throughout the series on her twitter account and this story shows that but also other themes as well. (some of these themes that I'm not a personal fan of but were still hinted at throughout the series)

Reading this story did give me some closure but I was already at peace and content with the ending by the time the English translation had come out, but I do hope that reading it can put your anguishing heart at ease.

I personally found the idea of Yousuke living with Iwaki to be okay but it was the other things surrounding it that made me either uncomfortable or pissed off.

I honestly didn't like the inclusion of Shou and his boy toy. (I'm obvious not a fan of them) I didn't feel comfortable about Youka showing the pseudo pedophilia moment that occurred in the series (a moment that was a exclusive chapter for either ALIVE 2 or 3)

I don't think Youka was ever trying to replace Katou with Yousuke. I am making the assumption that he was always meant to represent the child that Iwaki ALWAYS wanted with Katou. That theme and symbolism is seen in the chapter that Yousuke is born and represented in through the special connection the two share. (Which could also be a hint to the reincarnation theme of the series)

The special connection can also be a reference to the special bond between Iwaki and Katou. Not focusing on the problematic factors to their relationship, especially in Afterward, Yousuke is meant to be Iwaki's anchor. Yousuke knows that he could never replace his uncle (the memory of him spending time with Katou) but also knows that Iwaki would never see him as such and would always see him as an extension of himself (family) and Katou. To the fandom, mostly, and me, Yousuke is Iwaki's baby more so than his own mother.

I could be wrong but, Iwaki either breaks the cycle or reinvents the cycle by continuing to live which leads to them meeting again in the afterlife. How I personally view it is by thinking of the finale of The Good Place. I won't spoil the show but Iwaki finding peace and meeting Katou again in the afterlife reminds me of that shows' finale.

In conclusion, Yousuke was never meant to replace Katou but a symbol for a hopeful future and better tomorrow and Iwaki's anchor. I hope this answers your question. 😅👋🏾🙏🏾