We have managed to stay ahead of the costs of debt for two reasons.
1. There has been unprecedented increases in productivity for the last century.
2. The United States has been shown to be the most stable and safe country in the world. (You might argue about Europe, but Europe’s stability has been far less than the U.S. (WWII) and post WWII the stability has been made possible by the U.S. Military.
We cannot be certain that those two factors will continue for the next 20 years.
Re: “We have managed to stay ahead of the costs of debt for two reasons.”
You forgot to mention an imperative third reason.
Record low interest rates.