I don't disagree with you there. Some of these things are perniciously difficult to substantiate. However, in the case you just mentioned, I think you did a decent job of substantiating it. George Bush is indeed contrary evidence to the assertion that people don't hire blacks due to inarticulateness.
Some things for sure are really difficult to substantiate though. But what is the alternative? Ultimately, to enact change, you need to convince some portion of the culture that it's the right thing to do, or that there's a problem in the first place. I think there's a lot of evidence that efforts like that have been successful over the long term in a number of different areas.
Odds are good, you won't get credit. If you try to take credit, people will claim you are crazy, that isn't how it happened.
Sometimes, you need to decide what you want more: To make a thing happen or to get credit for making a thing happen.
Then you sit around wondering: if a tree falls in the forest surrounded by a crowd of onlookers who all vehemently deny its very existence, did it really fall? When you look again, will it be the same as it always was?
It is a little like arguments I have heard that black Americans who sound obviously black can't qualify for a job because they aren't articulate enough and it isn't racism. My response to that is "George W. Bush."
Maybe not hiring "inarticulate" blacks isn't racism. But why can an inarticulate white guy get a high ranking job if being articulate is so freaking critical?
There is always some excuse or justification for an overall pattern of behavior. Yet, the pattern persists and trying to point out the pattern gets dismissed for various reasons. It is a real life version of the trope Status Quo is God.