The threat to this is a collective decision of the indebted not to play ball. But this is as unlikely as a majority of bitcoin nodes subverting the blockchain.
You may have jumped ahead of yourself here Duke. The collective decision of the indebted not to play ball is the extreme end of things. In between society backing (law-abiding, ability to pay) and not playing ball is a lot of festering and increasing inability to pay.
But there is one debt which is dodgy, I admit. It is the debt owed by the government. The ability of the government to honour its debt in goods and services stems from its ability to raise revenues from the economy. The unprecedentedly easy monetary policy does place this in some doubt.
I'm surprised you don't seem to make the connect between the ability of the government to pay its debts by raising revenues from the economy, and an economy that is not generating enough revenues from which to collect from?