Where do you get $55 trillion from? Or $14 trillion? M2 is $9 trillion. While it's true that most of that money enters the system in the form of loans, that does NOT mean that it's value is dependent upon it being paid back. It's value is dependent on the GDP, which is the collection of goods and services which, by law, are tradable for the supply of dollars.
By $14 trillion you are maybe referring to the government's debt. That is also not what drives the value of dollars, except to the degree that new borrowing increases supply. The prospect of not repaying the debt would only affect the value of the dollar because the economic shock would reduce the GDP of the country. But the main point of confusion here is the failure to grasp that the value of the dollar is derived from the economic state of the country, not the financial state of the government.