It amazes me that the same people who believe in ‘Space Aliens’ refuse to believe in angels...................
That actually goes both ways.
I think the bigger problem in a serious conversation about the topic is basing things from both assumptions focused on trying to put things into neat little boxes: aliens (as extraterrestrials), demons or fallen angels (demons and f the fallen would not be the same thing either).
One origin for "alien" is "alius" which means "other". So, other than human? Well, all three of those categories would pretty much fit that description. The other latin term is "alienus", or "belong to another". Again, who or what would be the other"? World, species, being, etc???
So, we then have the extremes of believers. One side is that there are masses of planets just looking at earth with their own designs planned for it who don't believe in the possibility of a supreme being or beings. On the opposite side of the chasm is the religious extremes who can only think in terms of what the bible says, or doesn't say, and also choose to ignore the many other religions that came before the Christian Bible.
The answer is probably somewhere in the middle, and perhaps a mix of both elements. The argument for the existence of entities from another planet, does not the negate the possibility that demons (assuming inter dimensional) or even fallen angels, are also at play; or vice-versa.
But what there does seem to be evidence of, is that throughout history, other forces seem to have been involved with humankind, with the degree of their influence and intent being an interesting topic for discussion.