Timber your argument about zero net flow would work in a closed domestic economy but US debt flows in and out of foreign hands too. The current monetary stimulus is injecting massive liquidity into an economy that is stagnating. It is increasing supply of dollars beyond what the demand for dollars can absorb through GDP growth. Money printing isn't the right term - it's skewed monetary policy aimed at supporting the multiple bubbles in the US economy with cheap cash in lieu of fiscal reform and domestic productivity growth.
Yes the US Treasury created the bonds but the Fed is releasing the absorbed liquidity back into the market at near zero interest in order to flog the dying horse.
