The percentage of tax revenue used to pay interest on debt is the main metric that is the problem. Japan is at 50% of tax revenue used to pay the interest on debt. If the interest rate were to double, Japan would utterly collapse. 100% of tax revenue going towards debt interest payments is game over for a country.
I think it is around 10% in the US. Not that bad.
Anyway, the catch-22 is if they use austerity to cut the debt, the tax revenues fall and the percent of tax revenue used to service debt increases anyway.
If they try to spend their way out, they increase debt and the percent of tax revenue used to service debt increases.
In reality there is something structurally wrong with the economy and a business cycle crash will clear out the inefficient businesses so new ones can take their place. Which is a painful process no one apparently has the balls to go through anymore.
Where are you getting your 50% from? Also, the difference between Japan and the US is that over 90% of Japanese debt is owed to Japanese citizens. So that money is recycled into their economy. In America, I think it is 67%.