Strong disagree. This is something people say to sound clever. It’s not true at all. The real cause is basic entropy and complexity are always increasing. Rarely does a project manager request a feature be deleted. This is compounded by most companies having a few good programmers and mostly mediocre programmers, so the codebase will tend toward the mean over time.

It's really not.

Software solves business problems. Code that meets its requirements needs to be left alone. Developers just changing code because they feel like it need some coaching. They're wasting everybody's time for reasons that amount to "I like it My Way".

So, with normally performing developers, the only code that changes is the code that needs to change. Good or bad only has an influence on the cost of a change.