I'm not completely sure. I think MIT's conclusions might be correct, they might be preferable to GPAs. I also think there might be other alternatives that aren't easy to implement, that require either a restructuring of how we do school or a better distribution of resources than we currently have.
One conclusion that MIT hints at (although it doesn't say it outright) is that SATs might be a better indicator of success across economic levels in part because it's harder to buy a better SAT score with money. Looking at things like extracurricular activity runs into many of the same problems as looking at Github repos during hiring processes -- a lot of people don't have time to do a bunch of extracurricular activities, and access to those extracurricular activities is likely highly correlated with socioeconomic status. It might be difficult to move in that direction when access to school resources varies so much between areas.
I do think the SAT could be improved -- I think one really easy way would be to change how it's administered so that it optimizes less for formal test-taking skill. The really good thing about the SAT is that it's a less school-specific measure than GPA. So a better alternative might be a version of the SAT that kept a standardized metric but that either widened its scope significantly or was administered differently.
I also want to put forward the idea that admissions might just be really hard, period, and there might not be an easy way to assess potential, and trying to figure out the easiest way to do it might be like asking, "what's the best way to teach a child to play an instrument in a single day?"
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One really important point that I want to get across: there is a difference between a measure being good and a measure being "the least terrible option we have at the moment" -- and confusing the two can cause real harm.
At the top of this thread I see the quote, "so much for that common, popular notion that standardized tests do not predict anything of value." And if that's somebody's attitude, then they're never going to find a better option because the whole thing is being approached through the lens of "see, we were right, this is a good metric."
I think a lot of criticism of standardized testing, IQ, coding tests for hiring, etc... is not necessarily trying to destroy everything, it's just trying to point out that many of these measures are really bad and they shouldn't be treated with the respect they're often given. I think that someone can very easily both have the position, "yeah, MIT probably should use SAT scores alongside GPAs" and the position, "people place way too much confidence in these things as an indicator of success."