If legacy applicants as a group are as well credentialed or better credentialed (credentialed as defined by the university) then why do universities deploy a legacy preference (defined differently at different schools) at all?
You seem obsessed with the Ivy League to the point where you’re happy that so much of admissions there is like joining a frat or sorority because it gives your son an edge. Wouldn’t your son prefer to compete on a level playing field without the benefit of his privilege? Wouldn’t admission under those circumstances be far more satisfying? I guess not to you.