It is quite simple, really... Hanako-sensei embodies the hopes and dreams of young boys and a few young girls, no doubt... so there is no shortage of desire for her to stay there... and other desires too.
You know... I love the hued "morality" this story has.
I have a personal gripe with the idea of "there are no black and white morals, but shades of gray", not the "black and white," mind you... the "shades of gray" part... life is not gray; life is a kaleidoscope of colors that reach hues well beyond our understanding.
Certainly, some things are pretty easy to quantify; this can not be used to justify being a total asshole and other horrific actions, but when I see the complexity of "human" interaction, I don't see just different shades of the same, but a mix of emotions, feelings, and rationale that have vectors, waves, and intensity. ( I am sorry if I am overly prosaic here, english isn't my first language, and I have the habit of getting "romantic" with words)
Anyway, back to the point I was going for... when this story started, I entirely believed Yasaka, then the Yamagami, was just an evil being, sure... I knew there could be more complexity to it; it could have been many other things, some of them not naturally evil and just got corrupted.
But seeing Yasaka's memories, I see how well crafted his(?) character is... we know now that he is a invasive species of snake, we know that in time he ascended to become a quite impressive yokai, and we can surmise the path that lead him to become the first Yamatagi Madara, and how he may have even been a benevolent kami, but got corrupted by the worse side of humanity, but that the seed of his better side is still there, because of the better side of humanity too.
Knowing this story... he will just let it go at some point, he will stay as Amane's guardian (a role he is already doing very well... mama snake XD), and maybe... even reconcile with Dara... that would be good.
But I do think that his head, which is sealed, may be a problem.
It was sealed/enshrined as the focal point of his worst characteristics, and may have festered into something quite nasty... worst possibility... the remnants of the consciousness of the impurity(the sister) that were purged by Dara but not destroyed could seep into it somehow and turn into a whole problem.
Edit: I almost forgot to say this... you have to be a special kind of dipshit to see the incredible sacrifice and effort a mother made to save her child, and see it as some sort of curse.