You're going a bit far with "no longer worth anything" but indeed, if the entire world holds a particular nation-state's currency, and uses it to exchange goods

You're going a bit far with "no longer worth anything" but indeed, if the entire world holds a particular nation-state's currency, and uses it to exchange goods and pay down debts, then it gives that nation-state tremendous power. Printing dollars is often considered a tax on dollar-holders: I print dollars, your dollars become less valuable as my new dollars are circulated into the market. Effectively, this means the global reserve currency's nation-state can impose a tax on the world, instead of just on its citizens.