"Disparate Impact" is not about fairness. It is about non-arbitrariness. Factors that are demonstrably job related for the position in question and consistent with business necessity are considered a suitable legal defense.
Agree - in this case I don't think it would be hard to find some study somewhere that supports the idea that experienced workers attrition at a higher rate than average when placed in junior positions.
You might question the validity of the study, but it appears there is enough information out there to make a case.
