You seem to think that the deficit is proportional to the debt, when in fact the deficit is the first derivative of the debt. There's no proportional relationship at all. That is, even when your deficit is small, your debt is still INCREASING.
The relationship is still there -- whether exponential or linear, the fact is that *both* debt and deficit become more irrelevant the further out you go, due to inflation.
The debt is always growing exponentially, but the inflation and economic growth overshadow it. The total debt in 1970? 350 billion. Total government receipts just last year? Two and a half trillion. So, *all* the debt accumulated up to 1970 can be considered irrelevant today.
40 years from now, total current debt will be irrelevant, as well.