> This debt is only ever re-financed, isn‘t it?

so what? Have they missed an interest payment? Is their economy unproductive and cannot pay the interest?

Having the debt means you are entitled to a portion of the economic output of the machine behind that debt. Why is it a must for the debt to be repayable in one go? Esp. if the debt was not all issued in one go?

each time a particular treasury bond is due in full, the US treasury has been able to repay it. Whether they repay it by issuing new bonds, or increase their taxes and pay it off, makes no difference to the holder of that bond.