You have to add it all together, because the government doesn't own the entire economy. It cannot lay claim to all of the output to service its debt. That one economy has to service the debt of households, businesses and all levels of government. You're telling me the only saving grace we have right now is low interest rates. You've predicted that rates will remain low because the economy will be weak, but we have historical evidence that rates can be high in a weak economy. In the early 1980s, we had an extremely weak economy, and we had extremely high interest rates at the same time to combat out-of-control inflation. Why would we risk that repeat?
Why would we risk the possibility that we would have unsustainable levels of debt going into future increases in interest rates, all on the flowery assumption that rates will never go up again?