The debt isn't as significant considering there is revenues that could pay it.
Tax rates in the 50s vs tax rates today. The lack of higher income brackets in the tax code, etc.
We've got plenty of money.
The US has had a wide-range of tax rates and tax policy.
Despite that, FedGov has never collected more than 22% of GDP for longer than two years.
One explanation is that the tax code affects people's behavior wrt taxes.
1) We don't have the political will to increase revenues (read: taxes)
2) We don't have the political will to cut spending. That is, politicians keep buying votes with new programs, and we keep smiling as we sell our votes to the highest bidder.
In short, you're over-simplifying the base financial system (i.e., The Fed), the broader financial system (i.e., banks and the fractional banking system), as well as the sociopolitical system.
There's more to this than debt in some amount and GDP in some amount, and saying the ratio of those two is all we need to understand and not worry.
btw, for bonus points, read the first three or four chapters from Lyn Alden's "Broken Money".