I was referring to the US National Debt - only increases, no principle gets paid to decrease it. Evah.
You are wrong again. I guess you are not involved in buying US bonds. Every bond is paid back with interest and principle. The Treasury just issues new bonds to pay maturing bonds.
Sooner or later US national debt will grow to a level at which bond buyers will demand much higher interest or not buy bonds at all. That has happened already in dozens of countries.