Some might object that “existential threat” is redundant and that Reagan had it right with his preference for simply saying “threat.” Threat embodies the possibility of harm, which could be seen as potentially embodying outright negation—i.e. of existence. One might well sense damage or disempowerment as, in essence, elimination. Isn’t “existential threat” really just a fancy way of saying “threat”?
To an extent, yes, and that reflects, again, a quest to juice the word up a bit, keep it fresh. English is full of examples of our having done just that in the past, yielding what today are set phrases entailing the same kind of lily-gilding. We often call something a “damned shame” which is not really all that “damned”—and besides, isn’t something that is a shame already “damned,” technically? Rather, we say “a damned shame” as a way of saying that something was a shame with a certain air of dedication, commitment. What, precisely, is “stark” naked, as if when just naked we wear a bowtie or socks? If overwhelm means what it does, then what did whelm ever mean? The answer: it meant “to overwhelm.”
People added the “over” for the same reason it now feels so right to add “existential” to “threat”—it adds a sense of urgency and color to a message that otherwise might go by less noticed. To wit, speaking is never a mere recitation of things and actions and qualities decorated by some greeting conventions and a sprinkling of slang. Language is about keeping people’s attention, making it worth their while to give you their sustained attention.