your entire point is based on that 10x productivity rule. it keeps popping up. Is it just someone's gut feeling turned into an urban myth or have real studies been done?
"Programming managers have long recognized wide productivity variations between good programmers and poor ones. But the actual measured magnitudes have astounded all of us. In one of their studies, Sackman, Erikson, and Grant were measuring performances of a group of experienced programmers. Within just this group the ratios between best and worst performances averaged about 10:1 on productivity measurements and an amazing 5:1 on program speed and space measurements! In short the $20,000/year programmer may well be 10 times as productive as the $10,000/year one. The converse may be true, too. The data showed no correlation whatsoever between experience and performance. (I doubt if that is universally true.)"
To contribute zero a bad programmer would have to just sit around and reddit all day or whatever. Once the bad programmer starts writing code he is almost surely a net negative since eventually someone is going to have to rewrite his code and unless the code is completely decoupled from everything else, the time required to do that rewrite will exceed the time it would have taken if the bad programmer had never written any code at all.