I'm sick of all these software engineering articles which are so vague and give no numbers, no hard evidence, and not even any very specific anecdotes.
Edit: I was harsh. What are you expecting to see? Software engineering has always been a bit squishy compared to other disciplines. There are simple metrics of things like code complexity or bug rate, but I've never seen anything that accurately and succinctly differentiates between a codebase that is a chore to work on versus one that is a pleasure.
Coupling is one big factor here. But the people writing the code have to care about it.