Like any loan, it's not just about the amount borrowed, but also rate of interest on that debt that creditors take into account in determining if they have confidence in that borrower's solvency.
Right now with global rates so low, the cost of interest on the debt actually sits pretty low by historic terms, despite the high amount of borrowing. Low rates is what is keeping it all glued together at the moment.
But if rates rise for some reason, for example if inflation gets out of hand, then I expect we'll enter a sovereign debt crisis where people suddenly decide that lending money to a bankrupt entity for less than the rate of nominal inflation maybe isn't such a good idea... and then people don't want bonds, yields spike, and we have a SHTF scenario.
Given that Japan has been experiencing deflation, there is no pressure on yields to stay positive, hence why they have the particular luxury of investors willing to accept sod all return on their debt, but that won't be the case in an inflationary scenario.