> Bad code drives good code out, because good code is easy to understand and change.

I call this the Peter Principle of Programming:

"Code increases in complication to the first level where it is too complicated to understand. It then hovers around this level of complexity as developers fear to touch it, pecking away here and there to add needed features."

http://h2.jaguarpaw.co.uk/posts/peter-principle/

Without concerted effort, any easy-to-change code naturally will be replaced by hard-to-change code.

exactly