Well, Tolkien pretty much left left the door wide open when it comes to "all-powerful beings"
and besides, this is only my speculation. after all, what did all of the evil things in Tolkien's work do after defeat? they slinked away and waited to be returned to their old strength.
for much of this, melkor was locked away and could not aid his failing servants. there could have been another lord of far greater power about. one that may have still been "fighting the good fight" even after Sauron's failure.
you never know with Tolkien...
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My time is at an end, for I have walked from Valinor to the Far-east where men have not gone for millennia. Demons have fallen before me. And now... I must rest...
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