Here’s what sticks out:
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?
As my anecdotes demonstrate, not all alum children are treated equally in the eyes of the admissions office. Legacy admissions as it’s actually practiced - a huge advantage to the unqualified kids of large donors/celebrities/alums who maneuver to obtain high profile alumni posts - and little to no advantage to other alums is unfair - even to the vast bulk of actual alums who don’t have the wealth or connections to play this game. (And here I limit my observation to the Ivies about which there’s much public data.)
My answer to my own question is that legacy admissions are designed to help only the few alums while giving the impression of helping many alums to the benefit of the university. See above.