His entire point is that the method of educating isn't working. It's not working because the audience is often actively opposed to being educated. They like thinking that racism is a non-issue and they enjoy that their willful inability to learn can be misrepresented as him unable to "prove" to them otherwise. They enjoy this as a license for them to continue the status quo. In asking them "what would it take" he would be given insight in to exactly how badly they refuse to learn. Their answer to his question is a lede and then the goalposts never stop moving. He'll never convince them because they are subconsciously resisting fact and reason. They like the world with the wind at their back. If the question was "what would it cost to convince you" their answer would essentially be "you don't have enough".