I see what you're saying, I just think they do it well. Course I came into fandom knowing Angels and shit were on the horizon, so it was never a "turn" to me, but I dunno. Even though the show mythology has changed, the monsters of the week and the...tone haven't.

And I'm impressed by how many monster of the week things they still manage to fit in, and keep things moving, without being like "Let's ignore the apocalypse to go do something random for a week." It could be summed up that way, but it doesn't feels like it

Anything involving demons is going to have to include more levels then just a simple boogie man in some shape or form, and I can think of a few other options to explain them as a whole that is so relevant, but like you said, this show is very...American, and Christianity is our biggest mythology. As much as I loved, say, American Gods, I had an issue buying that half those gods and pantheons would have EVER been worshiped here, so using Demons from like...Hindu for their over arching mythology wouldn't have fit. I think once they started making demon's such a big deal instead of making Azazel something "else", the path was pretty well set. And I don't think they could have gone on for TOO long without defining some kind of over arching...antagonistic force.