Looking only at debt-gdp ratios misses this point, from John Hussman:
Still, it’s precisely that short average maturity that makes the debt problematic from a long-run perspective, because it can’t be inflated away easily. In the event of sustained inflation, the debt would have to be constantly refinanced at higher and higher yields. Contrary to the assertion that the U.S. can easily inflate its debts away, it is clear that sustained inflation would create enormous risks to our long-run fiscal condition by driving interest costs to an intolerable share of revenues. At that point, any shortfall in GDP growth or government revenues would result in a rapid spike in debt-to-GDP (as Greece and other peripheral European nations are experiencing now). Prior to embarking on an inflationary course, the first thing a government would want to do is dramatically lengthen the maturity of its debts.
For the pointer I thank Andrew Sweeney.