Japan has a declining population and working population, actually this is true of China now as their families are limited to 1 child its inevitable that they have less people and their working costs are rising. (rising wages natural inflation) Japan will not pay off its debts as its not been invested just spent.
USA also has this problem but does have a rising population, it could be argued they have greater productivity in future to pay off the debt. Its unlikely any government will cut spending, switch to a surplus and keep taxes high enough for many years needed to pay off debt in excess of GDP.
Cutting spending means less government basically, theres a self bias they just cannot turn off their own source of power and the people wont force this requirement until its completely obvious as all the various spending schemes are popular until funded from peoples taxes directly instead of delayed by debt accumulation as we have now. Reagan did the same as Obama as Bush as Clinton as any president, they increase spending and debt regardless of politics.
The only thing to look at is a natural change forcing rates to alter, economics should be more about this then politics but current doctrine is about forced moves and their effect rather then normal interaction in trade
You guys still fail to see the nature of this system. Your population stastistics doesnt count here.
The debt is unpayable by design. The debt is always bigger than the money supply, therefor it`s mathematically impossible to pay it back.