You are wrong, we have excellent tests of potential. Read about, for example, about SMPY, where researchers identified kids with high potential in middle school, then tracked their life as they grew up and built their careers. The test they used turned out to be highly effective and predictive: inordinate high percentage of those identified as talented turned out to be highly successful in academic and professional careers. See

https://my.vanderbilt.edu/smpy/files/2013/02/DoingPsychScien...

What was this highly predictive test? It was, quite simply, taking SAT at 11-12 years of age. SAT is really good test, and is highly predictive of academic success, its biggest flaw is that it is too easy for those at the top of distribution, but that can be ameliorated by administering it earlier than typical.

> If you truly think that the top 2.5% of black students would be "out of place" at Harvard then you have a completely invented mysticism of what it means to go to Harvard.

If 2.5% of all black students were of Harvard quality, then something like 16% of white students would be, which is something like 3 kids in every high school class. Harvard can afford to be much more selective than that, and in fact it must be so to keep its brand.