"your argument about zero net flow would work in a closed domestic economy but US debt flows in and out of foreign hands too."
I have addressed this before. It doesn't matter if US debt is held off shore....
When a foreign investor buys US treasuries, they first have to buy US dollars. Those dollars are not created by fiat. Someone has to sell them. The transaction takes place and the money base remains constant.
In an accounting sense, the US economy is a closed system. For every debit, there is an equal and opposite credit.
