And it's almost completely survivorship bias.
The hundreds of thousands of people who did not get stronger through their struggles are not represented, because they are not here to present themselves.
The actual stats are pretty clear in that richer kids do better and are happier as adults than poor kids.
Hmmm... Funny you mention that. I'm better off, happier and much more appreciative of life the than majority the "rich" kids I've run across as grown ups- and I earn more, too. I wouldn't consider the subset of rich kids who inherited wealth to be successful at all, they're riding on their parent's coat-tails and have done nothing for themselves. In the end I'm far tougher, adaptive, creative, and decisive that those who grew up on easy street- so while I was brought up in poverty I'm a much better off and equipped for real life.
There is a silver lining to humble roots. Shame on those who diminish all things other than wealth.
In other words, ignoring that while some poor kids may win in "hard mode" doesn't change the fact that rich kids play life in "easy mode", and as such, it's much easier for them to win, and much more of them will win, any merit and hard work put into it being equal.
It's like showing some succesful despite the odds black man in 1960 to show that "see, Jim Crow and racism doesn't matter, it's all a matter of will".