Don't get me wrong: I think legacy admissions should be eliminated. And my understanding is that my university (Carnegie Mellon) has stopped considering legacy status as part of admissions.
But the answer is that legacy is a one-hop removed racial bias instead of a direct one, where the schools engaging in it can claim that it's based on a purely financial incentive and that it applies equally to all of their legacies. It's like money laundering for bias: Finding a proxy metric that happens to correlate extremely well with race but never explicitly mentions it. With the current supreme court, that laundering seems kinda likely to succeed.
You actually have it backwards. Your claim is that legacy admissions bias in favor of the predominant race might be true for a school that had race-blind admittance criteria. In the opposite case, however, legacy admissions bias against people of the predominant race (for the general student).
Since legacy admissions come first, schools which practice affirmative action have a heavy bias against the predominant race (because those slots are all filled by legacy candidate). Which means that if you're of the predominant race, you have next to no chance to be accepted by these universities... (I mean, everyone has next to no chance, but for people of the predominant race, they are discriminated against severely).
In general, though, college admissions are pretty terrible... Having spoken with someone who worked in admissions at one of these universities, if you have a bright kid, you're better off moving to the middle of nowhere to make sure they're the valedictorian, rather than trying to send your kid to a great high school where why might only be salutatorian. Why? For smaller schools they rarely take more than one student from that school in any given year, so when the valedictorian who filled out applications to 10 top schools gets in to all 10? The salutatorian doesn't...
But the answer is that legacy is a one-hop removed racial bias instead of a direct one, where the schools engaging in it can claim that it's based on a purely financial incentive and that it applies equally to all of their legacies. It's like money laundering for bias: Finding a proxy metric that happens to correlate extremely well with race but never explicitly mentions it. With the current supreme court, that laundering seems kinda likely to succeed.