Multiple Measures of the Same Thing

My understanding of the SAT growing up was that it was meant to identify kids with potential who didn't necessarily have good grades, live in the places one expects successful people to live, or perhaps look like one expects good students to look. There seems to be a broad consensus at this point across the spectrum of educational politics that the SAT does not do this. The response is to align the test to K-12 and college and career readiness standards. But if we do all this correctly, a student's grades in school, her K-12 standardized test scores, her performance portfolio (ha!), and her SAT scores should all say exactly the same thing. So why do we need the SAT?