[hywel]

"I’ve rarely encountered discussions of contagion."

This surprised me: contagion is a good metaphor because it is a compounding measure of the growth of the problem. Just like an interest rate (a compounding measure of the growth of debt).

Most senior developers I've met have considered the interest rate of the debt, which seems like it has been renamed here as contagion. Maybe I've been lucky to just know smart people!

From the point of view of explaining these concepts, I'd suggest keeping the metaphors consistent. Tech debt should have an amount owed and an interest rate, tech infection (?) should have a potency and a contagion level.