But the ammount of "money" in our system FAR exceeds the ammout of printed cash. You don't get the money to repay debt from other loans, you get it by trades goods or services for money in the form of a job or business. And most of what banks lend comes from customer deposits. The cash they put in circulation is more generally the result of customer withdrawal than loans, most lent money never becomes cash, it's generally in checks or sometimes just eletronic fund transfer certificates.
So, it IS possible to repay all of the debt?