I don’t think it’s true that using socioeconomic status as a substitute for race-based AA will achieve similar diversity, at least among selective colleges and universities. Within every economic tier, the general rank-order of academic accomplishment is always the same, with asians on top and blacks at the bottom. See, for example, this article in The Journal of Blacks in Higher Education:
"But there is a major flaw in the thesis that income differences explain the racial gap. Consider these three observable facts from The College Board’s 2005 data on the SAT:
• Whites from families with incomes of less than $10,000 had a mean SAT score of 993. This is 129 points higher than the national mean for all blacks.
• Whites from families with incomes below $10,000 had a mean SAT test score that was 61 points higher than blacks whose families had incomes of between $80,000 and $100,000.
• Blacks from families with incomes of more than $100,000 had a mean SAT score that was 85 points below the mean score for whites from all income levels, 139 points below the mean score of whites from families at the same income level, and 10 points below the average score of white students from families whose income was less than $10,000."
So if you try to substitute SES for race-based AA, you get more diverse SES but you get far fewer blacks because at each tier there will be far more asians and whites with similar backgrounds but better scores. Note in the JBHE article that this difference is so profound that children from relatively wealthy black families underperform poverty-stricken whites on standardized tests.
When I was on an admission committee (med school; not university, but same idea), we saw this dilemma every day. I came to the conclusion that race-based AA absolutely must be preserved if we want reasonable representation for black students in selective colleges and universities. I was deeply disappointed with the decision of SCOTUS, and I suspect few individuals not intimately familiar with what selection committees face understand how difficult it is to get proportionate race-based representation using anything but race as a criterion.
It’s easy to say standardized scores don’t mean much, but nobody in academia really believes that. For any given student? Sure. For the broad average, it’s simply not true, especially for any quantified pursuit such as STEMs.