Earlier quoted context omitted.
A bond paying 2% that matures in 10 years is worth less than a bond paying 4% that matures in 10 years. So you can buy back low interest long term bonds for cheaper.
So... What happens when you run out of people willing to hand you money until you actually pay them back? What you describe assumes someone is always willing to take the gamble on will you make good. That's one hell of an assumption.
Finally, and most critically, the Treasury pays back the bond in dollars when it matures. The treasury can also create dollars. Thus, the Treasury will never be unable to pay back a bond.