If interest rates are zero and inflation is under 2% per year then why not have the fed print money without borrowing until inflation is back at 2% per year? The money could be used to pay off the national debt. Businesses would have a predictable floor of a 2% inflation rate. The slippery slope argument against that sort of policy could be prevented by having the policy written into law. It's a fix where everyone wins, right?
That's essentially what Quantitative Easing was supposed to be.
However, we've known for about 80 years that it doesn't work in times like these, and like the 1930s, when demand is flat because of a massive debt overhang. The demand for money just isn't there.
In these times, trying to simply expand the money supply is like pushing on a piece of string.
Japan's been battling with non-responsive flat demand for many years now (though driven largely by demographics). Monetary stimulus has failed. Fiscal stimulus has at least steadied the ship.