I think you are looking at the wrong metric here.

It’s not about “how many successful people out there didn’t go to the Ivy League”, it’s about “how likely is it that _my kid_ will be successful?”. And the Ivy League really has a major impact on the latter metric. (I mean successful in the classic boring way here - completely agree that this is a very debatable goal, but that’s not my point)

But that's my argument exactly. Even if kids with Ivy League degrees disproportionately have successful careers, lots of non-Ivy League kids also go on to have very successful careers. One possible explanation is that the Ivy Leagues are just really good at spotting talent early and not that they are actually responsible for building the talent. Going to an Ivy League school might be an indication that you are smart, hardworking and driven. However, if you were not already that way, the school is not going to transform you into someone who is.