When I read through posts and then comments on [wealth/income] mobility in society I get the feeling that lots of people don't understand that inter-generational aspect of it. Mobility in society is less about me making a bit more or less over time and more about where I am compared to where my parents are.
If we really believe that wealth/income relate to achievement and ability then a high mobility is a defining characteristic of that. Low mobility doesn't mean better off individuals are pushed down by up and comers, it means that the children of better off individuals do better than the children of less well off individuals. Think of high mobility as a generational reset. Each new baby will end up as successful as their talents allow, no more no less.
There is no reason to believe that the children in the Walton family are any more capable than the children of school teachers or janitors. The reason the Walton children do well is because their parents have wealth, and the reason that the Walton parents have wealth is because Sam Walton had a lot of success. Someone could be born to the Walton family without the mental facilities to tie shoelaces and that person will be a incredulously wealthy individual for his/her entire life.
On the other hand if a genius is born in inner city Baltimore to a way below the poverty line black family then that child will have to struggle to survive and even if he/she does "get out" and find success that success is most likely to mean a college degree and $60k/year. Pretty good and remarkable considering the outlook, but in a "fair" society that strives for greatness then the Walton child would end up a janitor and the Baltimorian [?] would be a rocket scientist or CEO.
Now we can't have the level of mobility that resets everything...mostly because more successful people would never (nor should they ever) allow their children to be subject to such a system. Society may have been better off if W had ended up failing out of state school and living the rest of his life as a drunk in a trailer park working as carnival operator, but George and Barbara would never have stood for such.
This is why people use the term "upwardly mobile". We don't want to prevent parents from doing well by their children, but we do want to give children opportunities when their parents cannot afford to do so.
High mobility means that ability and drive can create success (and that sloth and idiocy can destroy it) even when mom and/or dad can't foot the bill.