any tests will always be gamed by those with the resources to game them. either ethically (through special tutors and private programs) or unethically (cheating).
Standardized testing is a good predictor. In my country people are admitted based on standardized testing only (country wide subject exams and sat-like test) or results of university-adjacent "preparation courses" and it works fairly well. The affirmative action is realized directly as a bonus points to your scores based on the background, which reduces the effect of ethical gaming.
The core issue is the pyramid shaped system where not being at one of these super places means that you are out of the competition for best work in general.
Do you want only rich kids going to college?
The system now is far more geared towards sending only rich kids to college than any national testing and admissions system would be.
If your parents are rich and have are around and can provide excellent schools, nutrition, love and care, pay for extra-curricular activities etc, you are far more likely to end up being "smart" and "have discipline" than someone who grows up in a family where parents are absent, they have to go to an inner city school, they can't afford school trips, instruments, extra-curricular activities, since they have to have a part time job themselves to make ends meet.
My point is, that SO many of the qualities we think that somehow kids got "innately" are actually purely products of luck and circumstance. The ability to "work hard" isn't a gift some are born with and some are born without, it's learned and modeled from our lives, parents, teachers, experiences etc.
If you never have parents that buy you books and encourage you to read, it's doubtful you will end up being "smart"
My point was, people love to imagine world where smart and discipline is mutually exclusive with "parents paying tutor" or other rich person perk. ImAnd if parents pay those, kid must be lazy or stupid.
It is not so. Even super entitled kid can be smart and work hard. If rich entitlement is just another advantage and so is tutor. Poor kids can be as smart as hardworking too, just without additional advantages.
Plsu, some people are not smart or hard working due to genetics. And damm they can be rich too.
Sure, there are poor kids from bad backgrounds who, against all odds, end up doing well. But these are outliers. In general, coming from rich families who throw money into your education, tutors, therapy, extra-curricular activities etc, is a MASSIVE advantage, and as such, one of the highest predictors of ending up at a prestigious college and high paying job is how wealthy your parents are...