The US is in a unique situation that lets it print a lot more money than other countries. At the end of the day, we will end this debt by printing money. Anyone buying bonds has to factor in the possibility that the US dollar the coupons are paid in will be devalued. It's not a "risk-free asset". It just depends on when and how the US will start printing money, and where that money will go. We did quantitative easing for years, but the banks didn't increase lending very much.
A good economy is when we should have been raising taxes and paying down our debt. But instead, the Trump administration presides over the biggest debt, deficit and trade deficit in our country's history. So it seems to me that the next Bernie administration will absolutely need to do large infra projects a la FDR, Green New Deal etc. because the recession may be severe.