The "right place at the right time" argument sounds like you could create a cohort of all the SV-based entrepreneurs who arrived there within a certain time period (supposedly the earlier the better?), and somehow all current SV-based unicorns would magically link back to those folks. If anything, I am observing a surprisingly small correlation there - the average founder of a top 100 YC company went through YC 5 years ago [1].

5 years ago is shockingly recent and well past the SV gold rush curve.

[1] https://www.ycombinator.com/companies/?top100=true