I suggest they never went away...they just hid. Just a suggestion...no proof.
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My thought is that other angels have left heaven for earth, but are not part of the original crowd that was locked up in the bottomless pit for procreating with mankind and possibly involved in genetic engineering. Certainly there were giants in Canaan when God told the Israelites to go and take the land as their inheritance from Him. “As it was in the days of Noah, so shall it be at the coming of the Son of man”... they will be back, perhaps as aliens from beyond our atmosphere... eyes wide open is my modo; fall for nothing, and consider everything...