>if it's all about human nature, how do you explain a dramatic recent change?

It's difficult to tell if you're trolling or if you truly don't get it. At no point did the parent poster claim that 100% of a person's likelihood to found a tech start-up was nurture.

Consider this situation: Nurture (U) and Nature (A) are both factors in a person's likelihood to start-up (L). If L = UxA, then a 37% drop in U would cause a 37% drop in L regardless of differences in natural abilities.

A good starting point if you're interested in the interplay between nurture and nature in human psychology is Steven Pinker's The Blank Slate.