If market participants think debt is unsustainable, and borrowing costs correspondingly rise, that itself can be the thing which makes the position unsustainable. BIS central bankers’ speeches  1  Corporate borrowing costs are similarly low.
Investment grade bonds in the US and euro area and Australia are yielding only around 100 basis points more than the equivalent government bonds and even “junk” bonds from these countries are, on average, yielding around 6 per cent or less. Some entities (Graph 1) have issued debt with 50 or even 100year maturities.