[gills]

With the assumption that 'debt payed' means 'product shipped to happy customers', I think that fourth quadrant is a bit of a red herring in that it's not so much a kind of technical debt that must be repayed; but more of a 'technical buyer's remorse' that is a function of good engineers who continuously reflect on the design decisions they've made.

[j_baker]

I think that Fowler would say that a design should be constantly evolving. That's the agile philosophy at work. Now, you can agree or disagree if you want to, but requirements change so quickly that it's difficult to not agree with him at least partially.

Therefore, it becomes technical debt when these good engineers reflect on their design decisions and don't do anything about it.