The True US Debt Burden

In macroeconomics, it is often taught that that the US debt burden (ie., the metric we should gauge how concerned we should be about the national debt) is the amount of interest paid on said debt divided by the measure of GDP during the same time period.   This makes sense.   We can roll over the principal of our debt and we can just use some of our growing economy to 'pay for' the interest we owe and it's 100% sustainable forever with no real cost to anyone.   As long as the growth in the interest we have to pay to the bondholders does not exceed our economic growth (it would have to be decades for it to really matter that much), we are right as rain.

But then, many textbooks, including the one I use to teach my macro class go on to say the best 'rule of thumb' therefore is to try to keep a constant or shrinking debt to GDP ratio - not interest on the debt, just debt.  This I have a big problem with.

The problem with using that as a rule of thumb is that it is not very useful, mainly, because it assumes that interest rates paid on the debt remain constant.  But as we all know, interest rates are anything but constant and in fact real interest rates have drifted negative in the past few years in some cases to the point that the government actually can theoretically earn money by going in to debt.  Even if that is not the case right now, the fact remains that real interest rates have dropped dramatically (thanks to the Fed) in the past 6 years.   This means that any new government debt is dirt cheap.   IE., interest/GDP aka the debt burden cannot be nor should be approximated by using debt/GDP as a measure.   If what you care about is the debt burden, and as long as the government collects such information, why would you need another measure of approximation anyway?   I mean, unless you are trying to scare people:

The CBO, as an aside, believe the debt/GDP ratio will fall for the next decade or so before it starts rising again, but it's also important to note that the rise precludes any assumptions about the investments we are putting in to the economy today.   So, really, predicting this sort of thing a decade out is a fools errand anyway.

But, why scare people, when you can present reality:

The reality is we aren't paying anything more on interest to our debt really now than we were back pre-9-11.   IE., thanks to a recovering economy and falling interest rates, despite our increased debt values, the debt burden is actually falling and really has been since the 1990s if you take a longer-term view largely due to the tech boom, the end of the cold-war, etc., which despite the bust, still has aided our long-term economy to this day:

Say it with my tea-publicans:  the economy drives the debt burden, not the other way around.