Over a multi-year or multi-decade time horizon, it just deteriorates. The oldest codebases I've worked on were 30 years old; the one at my current company is a

Over a multi-year or multi-decade time horizon, it just deteriorates. The oldest codebases I've worked on were 30 years old; the one at my current company is about 10 years old. People come and go, refactorings are started and stopped, bugs are fixed on a tight schedule, and the structure of code divided into nominal blocks slowly dissolves.