Why couldn't they? What would stop them from minting a $1 trillion coin? The US would quickly stop being used as a reseruve, but the fact that it is the current majority global sreserve isn't preventing them from mainpulating their currency. Some major things that makes the US dollar attractive as a reserve is the transparency of the fed, the stability of the currency, and the abailablity of the dollar (no capital controls and a trade deficit). If you cut out the first two it would have a very rapid fall from grace.
To sum up: The US doesn't have any restrictions on its currency because the are the maojor global reserve but if they don't manage the currency responsibly they will lose that status.
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