Like your PC? That started as a result of the drive to miniaturize transistors by the US military. Like being on the site? The Internet and a very substantial portion of the networking model was developed by DARPA, the US military research arm. No matter where you are (unless your a stone age tribesman somewhere) odds are military research helped out somewhere in your life.
This is a very similar argument that car-makers talk about when justifying F1 races etc - initially a lot of improvement to civilian cars occurred but from now on they are such different things that the "how does it help civilian life when all things are peaceful" is fractionally less and less.
Also if that cash was spend on other things - like Health research - raw science etc - what sort of kickbacks WOULD have they given - perhaps even better ones... "This came out of it so its justified" argument doesn't consider what could have come from it being channeled elsewhere. Assuming "nothing" is what would have otherwise happened is a little one eyed.
The power vacume thing isn't going to happen in a few years - to be on par to America even if they went out isnt just cash but time. A lot of the navy and air power the americans have is by trial and error trying out different thigns - heaps of navy excersises, trying out this and that in practice - to work out what is practical and not. Spending the same amount of cash in the short term and hoping you will get close - is not cool - it will not work.. its time AND money - but mostly a combination of both that has stabalised over ..... time such that hierarchies and logistics seem to settle and become predictable.
20 aircraft carriers + who knows how many support ships - a "navy fleet" is ridiculous.
And "If we dont protect you - you will get harmed because you don't protect yourselves" is not eactly right, america doesn't do this because it has a desire to be a police force for the world. Constant control over the sea / air etc from almost anywhere is a pretty loud trumpet often blown.
The USA does have a lot of options. Saying "Hey we owe everyone cash - fk you we will only pay 20 cents on the dollar or we will outright not honor any of it - and yeah we will take the hit of others not investing in us for x years" is a choice they can make - which will likely win BECAUSE of how much cash is to be made by trading in America, you will be hated by other countries, but hey money talks (yeah credit rating will reach junk status but not for long).
I know that's unlikely ... but still an option.
Sounds weird but it may just work - remember man on the moon project ? The good thing about America is that they can inspire their own populous with a super-project. "Yes we can".
Pick three super-projects ... and if ONE of them works and is owned by America its a world changer on a dime.
Not saying these are the ones ... but of this caliber
Cold fusion and or micro fission
Teleportation (of something practical - doesn't have to be people could be a stream of raw elements or something )
Low-cost-zero maintenance materials and or super materials (for whatever would vastly help maintenance costs / viability of certain other things)
Super-foods (ie eat this one fruit and it ALREADY has every single goodies a growing person needs for a well balanced diet, this other one is for pregant moms - this other one is for ... )
Or whatever the scientific community deems "Probably do-able if you through xxx billion dollars and about the same man-hours, until then ... unless we get an indirect breakthrough that eases up some fundamental / conceptual brick wall by pure fluke". Just like going to war - suddenly everyone's employed, and if any breakthroughs come of it (and one or two likely would) - it would set up America really well for x years.