Are people expecting Kaizaki to be a saint? Just because he's helping everyone doesn't mean he doesn't have worries of his own. He has a f*king trauma after seeing his senpai suicide for goddamn sake!
After being so involved with his friends, knowing that he'll disappear from their memories, if you were in that position, what would you do? It's especially more so that Kaizaki is truly a good guy at heart, that he doesn't want their memories to be empty because he was in them, that he doesn't want to continue that and thought the best solution was to distance himself. Since the very start, he had a high justice that could turn toxic and he's doing that by being sacrificial.
At the end, he's human. Stop treating him like some sort of saint. He has a limit, and he has a breaking point like everyone else. The fact that he held for this long is already amazing and like everyone else in this manga, he just needs support to help him see things right. After he helped others so much, this is the arc that he'll be helped.