The problem is, if you were a member of the military and JCS, you may be concerned once we hit $1T or $2T. Additionally, anyone who is critical of our national debt is clearly pointing the finger at Congress (who controls the purse), and the President. That's not the job of the Joint Chiefs of Staff.
So let's say we do consider the "debt" a "national security" problem. Does that mean the President can enact a line-item-veto, even though it has been declared un-Constitutional? There are all sorts of implications of linking the debt to "national security" that are bad.
As I stated originally, no one has ever gotten hurt physically as a result of debt (it is speculative and derivative harm).