The big difference between debt taking 18% of budget at historically high interest and 8% of budget at very low interest is that if the low interest ever become

The big difference between debt taking 18% of budget at historically high interest and 8% of budget at very low interest is that if the low interest ever becomes very high again (say around 9x what it is now, which has happened historically) you're talking about interest taking 72% of budget which would be catastrophic. 18% at the historic high would require 4X the historic high (an 80%+ interest rate, which we have good reasons for believing won't happen in the US).