Evidence? I know free market theory sounds like the most plainly obvious, common sense set of statements but you still have to justify your conclusions. You are asserting without proof that the market is full of completely rational agents.
To give an analogy, if you have access to all the food you want, but you voluntarily limit yourself to kumquats and bananas for arbitrary, irrational reasons, in the long run you will develop health problems.
Likewise, a business that irrationally limits itself to a subset of all available talent will, in aggregate, lose out to those who are not thus limited.
I admit to not having citations of statistical evidence at hand, although I have seen many publications in the past that indicate diversity of employee populations is correlated with innovation and success. I did not provide evidence because I believe as a Gedankenexperiment, the conclusion is self-evident.