In my time as an engineer, I've found that thinking of tech debt as financial debt also helps. There is the initial convenience (borrowed money) of using the debt-ed approach. Then there is fix cost as Bill Clark name it, i.e. how much to pay back the debt if it were money. The impact is akin to the amortization schedule, i.e. what is the cost every time. For normal money, amortization schedule is over time, but for tech debt it is over usage. The amortization schedule of tech debt is discounted over time, as with money, _now_ is more important that _later_.
Contagion is a great concept, and I think it is a better name than interest rate, as the debt will spread through the system, and not just linearly with time.
Tech debt is also multi-dimensional and not fungible like money, which makes it a harder thing to reason about.
But the good news is, in my opinion, that sometimes it is perfectly fine to default on some tech debt, and never pay it back, delete the code. Then taking that tech debt was a win, if the convenience was more than the amortized payments.