[edit] someone who graduated college with an economics degree please come and correct the following vague and possibly totally wrong perceptions I have as a subject of the American empire /edit
The value of a country's money is backed by a combination of how much they produce and how much foreign currency and assets from other countries they hold (euros, dollars, gold) they have on reserve. Only the US gets away with having no actual reserve ...because a combination of military might and cultural strategic dominance has allowed it to BE the reserve for everyone else. This is why it somehow makes sense for America's economy to be based entirely on consumption rather than production.
OP is right. Whichever superpower controls the levers of global trade is the one that can sell debt and enforce the currency regime.
Some of us think that it's a lucky thing that it's been America, rather than a more authoritarian power, who had held that control for the past 80 years. Europe would not have recovered from WWII otherwise, and be living behind an iron curtain. Anyone who controls global trade after America is likely to be worse from a human rights perspective.