[srgvd]

Well, the economy might sustain, but the pension system is surely doomed. Which means some drastic political fluctuations, as major (and active) voters will be stripped from their expected (and promised) wealth. And any political disturbance when you are a 3rd economy in the world with 200% GDP in debt might blow up in World's face easily.

If this debt is ever to be paid, this'll be surely done partly with funds now parked at US treasuries. And, of course, any economic issues in Japan will hit China hard as well - which is another major holder of US's debt. So, top-3 biggest economies in the World might go down in instance, sinking others with them.

Japan might try to compensate that by active foreign workforce infusions (like Europe do), - but they are a really closed society still, and their foreigners count is just under 2 mil (compare with 16 mil for, say, Russia, with comparable, though much younger on average, total population size), which is nothing.