STEM, nigga. Masters degree, full time professional employment and not impressed by your soft science bullshit.
Your scenario is retarded, sorry.
Okay... bit of a strong place to start.
Surely in a "collapse" scenario, people are sellinf furniture or killing each other for food, not worrying about the Fiat currency of the collapsed ancien regime?
Fortunately the US has far more food than it needs, at least...
nations, now selling dollars, offer loans to the US
OK this gets stupider now... so the US suddenly collapsed but everyone else is basically OK... despite the US's central role in global finance, the global guarantees of the US military and the countries that depend on our food exports?
And the other countries are not only OK but also now in a position to suddenly dictate terms because the US is apparently still involved in international mega-finance despite, er, having collapsed
...and Dollars are still apparently worth something, even though the US has collapsed...
...and the US is where Dollars come from so they can just print more - if they're collapsed it's probably better to print more rather than let foreigners rip you off selling you your own currency...
...and the US military's power is not a factor...
I agree that there are some mediocre people in terrifyingly important positions of power in the US these days. But the people who built this system are arguably the greatest statesmen ever - their only rivals are Victorian Brits and that's only in relative terms.
The US's position is still extremely strong and there is no realistic rival. The EU could have been but they fucked it up.