yes — do you believe demons are dead and come back as zombies pretending to be aliens from outer space? I don’t think you do
Demons aren’t dead because it’s not the nature of angels, fallen or not, to die.
Fallen angels, by all indications from Scripture, retain the power that they had before they fell. It’s just a matter of whether they are good or evil.
Angels are known to have made appearances to humans many times in Scripture. Demons manifest through people as demonstrated in the Gospels. Satan can appear as an angel of light for the purpose of deception. Since he is a fallen angel, there is no reason to think that the rest of the fallen angels cannot do the same.
It’s dangerous to not recognize the reality of the spiritual realm. Not being able to recognize it leaves one open for deception by it.