I love this analogy, interest rates on tech debt. It's such a natural extension of the phrase tech debt and succinctly gets a point across. I'm gonna have to steal that and start using it at work.
The metaphor of interest was the whole reason that Ward Cunningham initially coined the term "tech debt":
"Technical Debt is a metaphor, coined by Ward Cunningham, that frames how to think about dealing with this cruft, thinking of it like a financial debt. The extra effort that it takes to add new features is the interest paid on the debt."