With a national test like the SAT you get compared to your peers, not against your classmates at podunk high. Bottom line, getting rid of the SAT is a bad idea, and is really only about increasing the power and discretion of admissions officers to do the social engineering they so want to do.
I did horrible on the SAT, but I had great grades from a good high school in Dallas. I also took hard classes in high school.
I got a degree in computer science from Texas A&M. I was originally a chemical engineering major. Most of the class flunked out of chem E after their freshman and sophomore years. These were even National Merit Scholars. I never flunked out (but I did change out of Chem E my junior year because I hated it).
I know that I had to work hard in college, and the people that didn’t work hard didn’t do well.
And yet, contrary to your assertion, high school grades remain a better predictor of success than the SAT.