I've never understood, maybe someone can explain it to me, why there's any concern about US treasuries? A bond holder has loaned them $1000 USD, and they liter

I've never understood, maybe someone can explain it to me, why there's any concern about US treasuries? A bond holder has loaned them $1000 USD, and they literally have the ability to print USD money. How could the bond holder not get their USD? Of course a loss of purchasing power may occur with printing, but it seems there should be a 0% risk of non-payment?