The point is that the schools retaining legacy do see a value in retaining that preference, whether it is for development, alumni engagement or some aspect of campus culture. That is their judgment to make unless that preference violates some law. People may see that as a negative force in perpetuating privilege for the few, but I think most of the selective colleges that practice legacy admissions also actively seek diversity. You can see the change in demographics over time and in many cases the generosity of need based financial aid.
If you believe that holistic admissions schools are building a class based on “buckets”, then its pretty logical to assume that freeing up a space taken up by a legacy will just open up a space for a similar candidate whose parents did not go to that school.