Don't legacy admissions often mean that their parents have donated significantly to the University? In a sense wouldn't that just mean that those students are essentially paying a very high premium to attend there and subsidizing the education of all the other students? I'm not against abolishing them but I do wonder if this will have any impact on alumni donations.
Large alumni donations can and often do increase the cost of education of all the other students. They frequently lead to large capital investments that then have uncovered operational costs. I used to work for one of the wealthiest universities with 100s of millions in capital improvements annually that couldn't provide functional HVAC and a fresh coat of paint to the buildings it had.
There's a lot of room to debate on what colleges should spend their money on, and what they should be providing. But American universities are not strapped for cash, and should be spending more of it on things that directly benefit students and researchers.