This change happened in five years. Such a policy change would be extremely popular. For proponents of race-based affirmative action, this would serve as somewhat of a proxy due to the correlation between race and class. For opponents, removing legacy preferences makes admissions more meritocratic.
The only groups with something to lose are (i) mediocre children of alumni, and (ii) the university's endowments if alumni decrease their donations. I don't believe (i) is a strong factor in a university's calculations, which leaves (ii). I believe universities are happy to see the public fighting about race-based affirmative action: indeed, dividing and pitting the low- and middle-income groups against each other over their limited subset of the pie, means they will not be working together to increase their overall share of the pie.
(I think this issue could and should be considered orthogonal to race-based affirmative action. Race-based AA was never put on a stable enough legal footing and will continue to bounce back and forth in the courts until it's resolved, while this could be resolved quickly and uncontroversially.)