Sure, and racial discrimination is presumptively illegal. Affirmative action is the acceptable carveout to this presumption. These no presumption of legacy admissions being illegal, however...
Well this is likely to change. Affirmative action is zero sum: it’s impossible without discriminating against some group on the basis of a protected class.
Not sure that’s the definition of zero sum. Presumably it’s correcting for some existing discrimination. Maybe it over corrects, maybe it under corrects. Highly unlikely that the balance remains the same afterwards.
Note that this is pretty much an urban legend, with little to no basis in law. "Affirmative action" when originally introduced as a standard was very clearly defined as acting to ensure that racial discrimination would not occur. Non-discrimination is, and has always been, the overall goal.
"Affirmative action" originally meant just that it wasn't enough to say "okay, we won't discriminate" - employers had to take some active measures to ensure discrimination didn't happen, but it didn't prescribe particular measures. That sounds vaguely reasonable. But over time affirmative action warped in most peoples' minds into establishing racial preferences.