Your principal is safe only if you demarcate principal in dollars. This is only a reasonable way to measure principal when inflation is negligible. With inflation widely exceeding interest rates, it is unreasonable to consider the only risk of US debt to be default and you have to consider the inflation loss.
Another angle is that all of the military defense backing the USD is coming from dilution of the USD (monetary inflation), or at least that is true as long as we continue to run a deficit.
Another angle is that all of the military defense backing the USD is coming from dilution of the USD (monetary inflation), or at least that is true as long as we continue to run a deficit.