As the Supreme Court determined in Dartmouth College v. Woodward, the private American college does not exist for some ambiguous public improvement project; it exists to do as its trustees see fit. Part of the reason these schools can afford to be so generous is because they have devoted, generous alumni willing to give to make their school great. There’s no evidence that any college will enroll legacy students who aren’t otherwise prepared to succeed to college, but colleges have to keep the wishes of its generous graduates in mind.
That means the institutions can give out scholarships and build new buildings, it also means keeping alumni happy. An easy, and cheap, way to do this is to give legacy preferences. As long as no one gets hurt (and no one really does) the preference for such applicants makes a lot of sense. It’s time to let this one go.