There is nothing wrong with Isumi. His side story is indeed interesting enough. However, it is not what one desires during the story when Hikaru is attempting to find Sai.
Perhaps this can be seen as an allegory for depression or being trapped in a rut. It shows how sometimes, in the midst of our search for something important, we are diverted by other things that, while interesting, are not the main focus.
But I really hate the fact that we have to lose Sai in order to reach this point. Sai is such an important character, and his loss feels like a huge blow. It makes one wonder if there was another way to tell this story without having to sacrifice such a beloved character.
Nevertheless, this is the way the story has unfolded, and we have to accept it. Maybe there are lessons to be learned from it, about the importance of staying focused on our goals and not being easily distracted.
Or perhaps it is a reminder that sometimes, in order to grow and move forward, we have to let go of things that are dear to us. Either way, the loss of Sai is a significant moment in the story, and it will surely have an impact on the readers.
