Look at this video and tell me how that is sustainable: http://www.youtube.com/watch?v=-quUyId2WZ0#t=0m13s
Why will Japanese people buy government bonds that return a negative interest? Stuffing cash in a mattress seems like a better option at that point.
The only number the government does not control is the interest rate they need to pay.
That makes it unattractive for speculators to bet against the country. Countries like China or Germany are in a similar position.
Japan currently funds more than half of their gov't via borrowing, with the demographic shift and the internal demand for JGB turning towards net negative, they will increasingly rely on external buyers of their debt. I believe China has been a buyer of late.
What has worked for Japan historically may not work so well going forward.
In a sense, this system of internal borrowing in a hidden tax. While it of course is not mandated, it's required in order to keep things going smoothly.