I mean, it is clearly the case that ultra-selective schools are benefiting a lot from the preexisting quality of their students, and they wouldn't obtain the same outcomes by taking a random high school senior.
But the students who apply to Harvard and fail to get in probably go to schools that are only a little less prestigious than Harvard, instead. Like, I didn't apply to Harvard, but I did apply to Princeton -- and got rejected. I went to Williams College instead. If my income is similar to a Princeton's student's post-graduation, is that because I did it all on my own, or is it because Williams is also a highly prestigious school that provides similar benefits to Princeton, albeit perhaps a bit less?
Also, without context, the smartest people at Harvard could end up being career poets, while the rejected applicants could all end up as MIT engineers and Wharton business majors. While an extreme example, even the elite universities have academic focuses, attract students interested those, and accept students based on how strongly they want those interests represented in the student body.
But the students who apply to Harvard and fail to get in probably go to schools that are only a little less prestigious than Harvard, instead. Like, I didn't apply to Harvard, but I did apply to Princeton -- and got rejected. I went to Williams College instead. If my income is similar to a Princeton's student's post-graduation, is that because I did it all on my own, or is it because Williams is also a highly prestigious school that provides similar benefits to Princeton, albeit perhaps a bit less?