I disagree, somewhat :-) There are people whose contribution is horrible but it's rarely confined to the quality of their code. It's more about being able to lead the company in directions that are ultimately wasteful in some way or other and not being able to change direction when the time is right. That leadership may be expressed in code, but the code is as much or more a symptom than the root cause.
With basic QA practices in place (peer review, coding standards, designs grokked by teams not individuals etc.) a single person usually can't muck up the codebase very badly. He or she can still poison a whole project or company.
Well, my understanding is that developer's experience changes the shape of the probability distribution of excellent/horrible contributions. But it is always a distribution. And that it is difficult to accurately distinguish between excellent/horrible contributions without hindsight.
I find that as I get more experienced, I'm more aware of how much of this huge discipline I don't know, and how many mistakes I've made in the past, and I'm much less likely to call myself a "good coder".
When I was younger and slapping shit together any old how, I thought I knew it all.
With basic QA practices in place (peer review, coding standards, designs grokked by teams not individuals etc.) a single person usually can't muck up the codebase very badly. He or she can still poison a whole project or company.