I think it's marginally useful to think of low vs high interest debt in terms of technical debt. I can make assumptions that certain areas of software will not change in the future, but that in itself becomes technical debt. Now future features have to be designed around the idea that certain features are off limits. Sounds extremely risky to me to try to hedge your bets on this kind of foresight. On the other hand, I think it's extremely useful to examine different libraries / modules as having different levels of technical debt, but the metaphor stays intact. You just look at your software not as a single loan, but many micro? loans.