This makes me wonder if anyone has done thorough (and replicable) research into which amount of strife leads which percentage of people to become more well-rounded rather than bitter and traumatized.

I agree that zero strife is probably not the optimal amount. OTOH, there are many people who manage to work their way up out of the ghetto but are definitely not well-adjusted. The agressive paranoia and constant need to prove their "big-ness" are very telling. You can see this in a lot of war survivors too, the constant suffering and uncertainty just breaks people in ways that are almost impossible to recover from.

I completely agree. People only grow when they step outside of their comfort zones, whether intentionally or not. But there are many more ways to do that than starting your own business. I suppose doing that provides an even greater dosage of that than most other things.