When the federal reserve can print the money to buy us treasuries, or persuade giant banks to hold excess reserves in us treasuries by paying them a premium, there is no limit to how much can be conjured up and spent. Japan is on qe 9 or something and no plan to slow. Europe is expanding theirs as well. We use the interest on the feds 4 trillion balance sheet to buy more treasuries or direct at congress for spending. I agree it's unsustainable and will fail at some point, but there's no way out unless we default(cut gov spending, take tax revenue with it since most jobs/corporations depend on said gov money), inflate it away, or keep the game going as long as possible.
John meynard keynes was well known for his theory of aggregate demand, when a government spends more money the gdp and tax revenue increases correspondingly if you start cutting gov spending the gdp begins decreasing(depression) there's no way out..
The national debt will increase forever in perpetuity, and will never be paid off, no country since the dawn of time has paid thier debt off, they can only default and start over with a new curency. It's the reason behind the gold standard that has long since been abandoned.