Ik this comment is late AF but the meaning of “purity” didn’t mean the dragon ascended. He and the other dragon wanted their race to become extinct. So that was referencing him dying and the realm or whatever being “purified” by no longer having the scheming POS dragons in existence anymore.
As for the reason the angels and demons didn’t go to Earth, it’s simple: they went back to wherever they came from. Presumably, they were called to that world just like the humans were and now that that world doesn’t exist anymore, they were sent back to wherever they belong like the humans were sent back to Earth.

Wym purity ? He ascended or what lmao?? Anyway it seems all of them came to earth except the angels and demons?