A factor for devaluation must be included in the equation. So long as the devaluation rate equals or exceeds the interest rate the money is free to the borrower.
The debt total is decreased annually in real $$ more than the interest owed
My view is around 7%. A higher rate of devaluation will not be tolerated for long.
I can't write an equation to determine at what interest rates the Feds would cease borrowing. The number of variables would be enormous. It would be academic at best because the obligation to service the debt is not a matter of choice.
Setting that aside let me see if I can understand why I agree with your observations.
I would agree that devaluation of the dollar must be included. However, it can become tricky for a layman as myself.
Dollar value is a function of existing dollars.
Existing dollars is a function of debt.
Debt is a function of borrowing.
Borrowing is a function of inflation.
Inflation is a function of how many dollars exist.
A circle if you will.
Introducing spending into the equation provides a control variable resulting in a solution, which is, spending controls inflation.