[wpietri]

To me, "systems accrue debt" is a consequence of three things: entropy, change, and learning.

Entropy is pretty obvious. Change is about the divergence between the system and the world it's reacting to. E.g., a payroll system that was fine in 1960 would not be fine today; our concepts of work and employment have changed significantly.

Learning is the most subtle thing for me. Knowledge is basically a ratchet: this year we know more than last year, and so on back. A software architecture that was brilliant in 1970 was outdated by 1980, and so on for every decade. As years go on, we learn more about good ways to build things, revealing previously unseen flaws in old systems.

One could argue that this concept of debt is unfair, that it's a moving target. That's true in some sense. But systems are things we create to improve the world, and the world was never going to sit still.