This manga really makes you think despite it being so short, huh?
Himeno's case is interesting, but it also makes sense since her and Kusu were only prodigies in elementary school. But then imo, being smart in elementary doesn't mean jack. It's like easily completing level 1 of a video game and...
You know, I wouldn't be surprised if this happened to some of my former friends whom I've gotten along with from high school. There are simply too many possibilities for each individual.
I don't like thinking about it though.
It's crazy real and sad how individuals can just vanish out of your life and never be heard from again if the gap in conversation is too long.
They just disappear back into the sea of 7 billion people, almost like all those times spent in the past were simply poofed out of existence and gets reset.