> In a fair society, with equality of opportunity, it should be equally likely for a child born in a trailer park to become high income earner as it is for Bill Gate's child. That is the normal social mobility we should expect in a democratic society with no class structure. We are so laughably far off from this everywhere in the world...

That’s only if you assume there isn’t a generic component to traits that influence (broadly defined) success. But of course that’s not true.

We should expect Bill Gates to be much more intelligent, conscientious, and to have much better impulse control than typical people living in a trailer park. Ditto for many other traits. Therefore it’s much more likely that his children posses these traits compared to children born in trailer parks.