It's not an assumption I'm making at all. It's an independent variable. I'm assuming that talent is more or less uniformly distributed among applicants. If that assumption is true, then an optimal hiring process will, with sufficient hires, produce a workforce that broadly resembles the pool of applicants.

More formally, under an optimal recruitment system, as the number of hires approaches infinity, the Levenshtein distance between the anatomical characteristics of the workforce and applicant pool approaches zero.