The industry is unable to provide an acceptable boundary for pilots' contributions to safety; instead it is sidelined to another judgement of 'good enough', but this is difficult to judge particularly if the boundary of human performance is unknown, or worse still believing that humans should not fail.
When measured against the aviation 10^-9 criteria I wonder where pilots fit. Sort of a MTBF if you will. Personally I reckon they do a pretty good job.