Sounds like a good reason to eliminate legacy admissions if the claim is that these students can compete without the crutch.
I was not personally a legacy admit to my college (so no skin in the game really), but fwiw I don't think it's an entirely illegitimate thing to select for. most schools have their own distinct culture and consider that an important thing to preserve. I imagine families that try to send their children to a specific school over multiple generations probably feel a stronger connection to that school than the typical graduate, so putting a thumb on the scale to help them seems like an approach that could work.
if anything, I'm more upset about how recruiting for college sports works.
Do you have data supporting your claim that "plenty of legacy admits are diverse"? By definition the legacy admissions are very similar to previous attendees. You need to explain how that would increase diversity.
Just test scores and GPA have never been the only factor and never will be. For example universities aren't looking for robots who are just good test takers. I'm surprised you're speaking on this subject but seemingly haven't done the homework and aren't aware of what existing practice is and what the ruling still allows.
Going forward almost nothing will change in outcomes. The ruling said universities are allowed to use a student's life experience of racism as a factor. Harvard already highlighted this in their public response to the ruling. That's not race but it is a very strong proxy for race isn't it?
Legacy admissions can only maintain the status quo so there will be no increase in diversity if legacy admissions continue. To increase current diversity levels you would have to put some effort into it. The status quo rarely changes organically when it comes to gaining or losing power.
Removing that would be easy.