You are right, the notion of the fallen is strongly espoused by Abrahamic faith, but not all demons are a fallen version of anything. The demons in Sumerian myths are not fallen. Nor the evil spirits in Slavic myths. Or those in Japanese legends. Etc.
Even in the case of "fallen" beings outside Abrahamic legends, some do exist anyway but that is straying away from the point which is that demons (and of course angels) exist in other cultures.
"the roles they play aren't usually exclusively negative or positive, bar the odd individual like Apep."
The point of a creature being classified as a "demon" is that it is a spirit that upsets the stability in some form (a function that some gods also have, hence why demons and angels are not "divine") so they are very much negative. They can choose to repent, which is a theme present in every religion, including Abrahamic faiths. It is not, focused upon, but it is a surprisingly common theme. Rakshasas of India, Shedim of Judaism, Oni of Japan and more can actively choose better paths, but it is treated as unique because a vast majority of their kind is dangerous and (dare I say) "evil".