> Because debt-to-GDP is apples-to-nonsense. No it's not. Debt is measured in dollars. GDP is dollars per year. Debt/GDP is $/($/yr)=yr This ratio converts debt, a number it's hard to have intuition for, to years. It tells us how many years of productivity we owe. For those of us who don't manage $30 billion in assets years of productivity probably carries more meaning than big numbers with 12 zeros.
Current interest rates can give a somewhat distorted picture for the US. Banks buy US debt because they have to due to regulation, other countries buy US debt as part of their economic policy, the net effect is that the US government is able to borrow money more cheaply that it should based purely on credit risk. Even so, the US is able to borrow money at historically cheap rates.
My understanding is that if current trends is health care costs and economic growth continue then in the long run the US will be unable to service its debt and maintain its current spending programs without raising taxes. Most spending is on the military, Medicare, Medicaid and Social Security, so there will be some difficult political decisions. However, that doesn't mean the US is close to a debt crisis today.