As far as needing a "clear existential threat to help justify our gargantuan defense expenditures", perhaps you missed the part where Bush I and Clinton together reduced the American military by half because the Cold War was over.
It's tough to reason with people that live in a world fabricated within their minds based on huge doses of fantasy in which everyone in the world loves one another and wishes nothing but the best for their fellow man. A world where everyone gets along and shares everything they have.
Somehow they think that if a nation adopts that posture that the world will respond in kind. They are oblivious to the fact than human beings are still tribal people with very strong desires for both conquest and survival. We're doing a fairly good job dealing with the conquest part (radical Islam is the exception) but the survival aspect has us just one disaster away from anarchy.
Until mankind is exploring other planets wearing funny clothing and not requiring any form of compensation, we're all essentially still at war with each other and defense is highly important. The cart, it can't come before the horse.
Oh, and even in that utopian scenario, defense will still be highly important because the little green men we encounter may be having a bad day.