If private investors purchase $100 billion in US bonds, that is $100 billion in cash that is not available to be invested elsewhere (i.e. in the private sector)

If private investors purchase $100 billion in US bonds, that is $100 billion in cash that is not available to be invested elsewhere (i.e. in the private sector). When government borrowing goes down the cash flows to other investments it doesn't just sit there waiting for Uncle Sam to change its mind. Nor is available cash created out of thin air just because the US government wants to sell more bonds. Obviously the size of the credit market changes over time but in the short term, public and private interests are competing for the same pool of available cash.