There is a fundamental difference between handing over money to the government as a tax that you will never see again and loaning money to the government with the expectation that you will receive it back, plus interest. But yes, as long as the government can still sell treasuries, it can continue deficit spending. The problem is that as the financial imbalance grows larger, trust in the government's ability to repay the debt wanes and, as with any borrower, the interest rate on new loans increases, which places further strain on the borrower's resources. Until, one fine day, the borrower can't borrow any more at any price. I have no idea how much government deficit can be financed until there is a buyers' strike on Treasuries and I'd rather not find out.