I think that your assertion that "unfounded prejudice is unstable in a competitive market for labor" has a tacit assumption that Gwen faced a competitive market

I think that your assertion that "unfounded prejudice is unstable in a competitive market for labor" has a tacit assumption that Gwen faced a competitive market for labor. For example, suppose other businesses will not work with you if you hire someone who is not "acceptable"? Suppose there are legal penalties (supported by members of the powerful group)? Suppose there is a union? My point is that that there are non-market forces which can strongly effect job selection.