For US treasuries the buyers are now waning. The most recent evidence points to the Fed as the main buyer of these bonds last year, with the Chinese second in line and then the banks.
Cash out of US treasuries today and you are paid back in devalued money. Money that is devaluing because the Fed is printing it. But the total amount of money outstanding is so huge that the market would instantly crash if everybody demanded their money back. That is indeed by definition a Ponzi scheme.
Why do the Chinese and banks around the world keep on buying US T-bonds? It is of course a matter of self-interest. If they stopped then the value of their assets held in treasuries would crash in value and the whole financial system tumble. And for US banks buying bonds with free money from the Fed is a money spinner.
If you look back at major financial crises in history then no serious crisis has ever ended without a crash in the bond market. It would therefore be far more surprising if the current global financial crisis avoided a bond crash than if it happened.