Colleges should not end legacy admissions because the advantage legacy students have is not due to legacy preference policies themselves. Legacy students do have a small upper hand, but it is usually due to the circumstances that allow them to be outstanding applicants; their legacy status does nothing to change this.
Legacy students are often the children of wealthy alumni; they often grew up with every benefit available to them such as tutoring and an array of extracurriculars. This is the reason why legacy admissions are at an advantage; it is because their parents are able to create circumstances and opportunities for them that allow them to be qualified applicants. Very few applicants who are legacies have no chance of getting in.
Colleges do not give legacy students admission to their school just because their parents are alumni, they give legacy students admission because they tend to meet and exceed the necessary requirements for admission.