All a US economic collapse would do is cause a reset in military obligations and posturing. We would be isolationist by default for a few years, while the economy re-built. There would be a power vacuum in some regions, and the security situation in Europe and SE Asia would be interesting, to say the least.
Dreams of the US disappearing or being occupied have been conjured by many a foe, only resulting in extreme disappointment.
Even if the standing military and nuclear forces vanished, the American public is heavily armed, and is full of generations of experienced warriors.