I've read that most of this money came from retiree savings, which are rapidly running out. Furthermore, the younger generation doesn't have such savings, yet alone the population, to keep such a thing going.
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.
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.