Thanks for bringing up your concerns. I mostly wrote it like that as a joke, but if it bothered you it will probably also bother others.
What do you think about: "That might have not been the case were they living in a world without monsters, but alas"
or: That might have not been the case were they living in a world without monsters, but alas it was not to be"
or: That might have not been the case were they living in a world without monsters, but it was not so."