I am not a trained economist......................

........I'm not even untrained.   But, unless someone cares to point out my mistake - in which case I will apologize, it is thinking like this that gives professors a bad name.   Post being commented on is here.  Lead off is here:

Why not worry about the deficit right now?

Because if I did my math right, Federal Government interest payments are at a long-term low relative to GDP.

 Chart is here:

Professor Green says, "Note that interest rates would have to double for us to be in the situation we were in the late 1980s, the end of the (ahem) Reagan Administration.  Do we ultimately need a steady state in which debt to GDP doesn't rise?  Absolutely.  Just not tomorrow."

Just two of many possible comments:

First, with interest rates currently at historic, and probably unsustainable, lows it is fairly easy for this professional borrower to imagine rates more than doubling in the not-so-distant future.  It wasn't all that long ago (1985) that 10% interest rates looked like a good deal.   When rates do move, they tend to move in a hurry.  Can you imagine the impact of the government's interest costs when rates get to 8%?

Second, doubling down on debt just because the interest payments are artificially low ignores the fact that sooner or later some, or all, of the principal needs to be repaid.   Focusing on low debt service (interest only) payments tend to obscure the lurking danger of every increasing debt.  

Debts, even governmental debts,  are affordable until, all of a sudden, they aren't.  Just saying.