It was literally unsustainable then as it was in 1933. Unsustainability, now that it is pure fiat, has to be defined in order to answer the title question. The definition of such, may differ from person to person or entity to entity. In my opinion it is unsustainable when the inflation from printing to pay the interest or rate hike recession causes social unrest. We are arguably in the midst of that right now, though very few people realize how their lives are being impacted by the printing of money. Stable means of exchange is the single most important factor in preventing revolution.

The fed is in a bind. Print and inflate or QT/rate hike and recession. They've been putting a thread through the eye of the needle. The eye is getting smaller and the thread bigger everyday. One day they won't be able to sew.

yup, people on online communities think Reagan screwed America up but IMO the real underlying cause was abandonment of gold standard. It basically provided a decoupling of monetary policy from underlying physical inputs like energy (gold traditionally used to be a proxy for that). once money was 'freed' the politicians & elites were empowered to do whatever as they can always print more. is it any wonder all the pro-corporate/anti worker policies & prison population exploded since the 80s.

Now something more controversial, in my fairytale ending I'd have expected proof-of-work type crypto currencies to have provided at least some counter balance to the fed money printer but in really bitcoin/crypto because more correlated with liquidity. I doubt that going to change ever.

For having hit unsustainable levels in 1971, it has sure held up well for the last 52 years. I would expect something unsustainable to do less sustaining.

(Now, true, the federal debt was unsustainable under the Bretton Woods arrangement.)