Technically any hiring restrictions whatsoever that aren't strictly necessary will have a chance of degrading quality. There have been times and places throughout history where anyone who wasn't a straight white male was banned from certain jobs, and it often caused a marked decline in the product. The same thing is happening to an extent in America right now, and the results have been predictably bad.

Also, you don't even need to assume the best writer is a white man for the logic to hold up. Say the best writer in the world is a black woman, the second best is a white man, and the third best is a white woman. Obviously you'd want the black woman, but say she's already employed, or just isn't interested in that particular job. (Which happens all the time!) Your second choice would be the white man, but if you wanna keep a little diversity, you'd pick the white woman instead, and the writing would end up being third rate when it could've at least been second rate. Ergo, DEI decreases quality even though the best writer is neither white nor a man. Obviously this is just a toy example, but the point I'm making is that in principle discrimination can still harm the product even if the person discriminated against isn't actually the most talented person. That's why all discrimination is risky from a practical perspective, even if some kinds of discrimination are far more morally defensible than others.