That debt represents a trade Japanese people made: they get to take away 100B of US production to enjoy their old age. In trade they gave the US government some

That debt represents a trade Japanese people made: they get to take away 100B of US production to enjoy their old age. In trade they gave the US government something. Oil. Toyotas. Whatever. The US government helpfully made that trade, promising US babies would, once grown up, deliver on that 100B of, say, labour. The real problem is that neither the US government nor (now grown up) US babies intend to hold up their end of the bargain. And vice-versa.