Yuki is the only reason they're not just super depressed at the end of the world.
I think it is way more of a stretch for high schoolers to sensibly be surviving like they are without going a little crazy.

Like, Miki's friend left their safe haven because they didn't have someone like Yuki.

I'm not saying it is a good thing that Yuki is kind of crazy, but it certainly helps to have a cheerful person so you don't just give up since there isn't anything to live for.