A bit on etymology—the word demon came from Greek and originally denoted, as we know from Plato's records of Socrates, a side-child of the gods, but not from humans, rather from lower mythical creatures (nymphs, etc.). A demon was a positive being back then, but Christianity, in the process of its implementation, used such a dishonest method as the discredit of its opponents, and thereby declared all mythical creatures—and demons along with them—to be evil.
But that' same, just a little reference:)
As for removing these races, I believe it is necessary for two reasons:
1 - in theory, they should be disproportionately powerful
2 - their existence (especially angels) may be offensive and unpleasant for religious people
PS - but do not take these words to heart...
- Чжуан-цзы
Сильванис