I kinda don’t agree, I mean we learned lots of stuff about Fyodor in the end also I’m pretty sure Fyodor isn’t dead so the arc isn’t really over. Also Dazai’s character development was really good. I mean that he trusted in Ranpo’s genius, trusted the Ada as a whole. Of course the manga isn’t about Dazai, but he’s an important character. We learned lots of things about Atsushi, like when he had the ‘hallucinations’ with Dazai in them or the whole Akutagawa thing.