[wrs]

Old large codebases are mostly maintained by people who weren’t around when the code was originally coming into existence. They don’t know the implicit design assumptions and decisions, or even the history of requirements. One thing you’ll find in nearly any software project of any age is a lack of good documentation of those things, so as you lose the community folklore, people will start making myopic changes, cargo-culting, violating future-looking design principles, and so forth. Pretty soon you just have a pile of incoherent features and making systematic improvements is hard because the code is no longer systematic.