Sounds like the 'bad currency drives good currency out of circulation' problem. 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."
I will highlight though that I have worked with people that choose the path of least resistance regardless of complexity of the code that would need to be changed. It's as if they refuse to read code.