No, a clear, objective measure on whether our debt is or isn't sustainable would require a benchmark debt to weigh against. In other words, if someone chooses n

No, a clear, objective measure on whether our debt is or isn't sustainable would require a benchmark debt to weigh against. In other words, if someone chooses not to invest in American debt, then will instead invest in other debt. The relative risk and attractiveness is where the cost and therefore sustainability comes from. IOW, you can not buy American debt. But you would instead buy Chinese, or EU, or Indian, or Russian. There is no "pure debt" to buy as an alternative (gold isn't the same).