You are introducing an additional variable that doesn't make any sense to include. Even if mobility is greater now than it was (something of which I am skeptic

You are introducing an additional variable that doesn't make any sense to include. Even if mobility is greater now than it was (something of which I am skeptical), the fact remains that the top quintile is still only ever 20% of the population. If one new person moves into the top quintile, that means someone else has moved out. "Most people's earning potential goes up over time." This is generally true, but it goes up like compound interest. If you start high, it tends to get massively high by the time you retire. If you start in the middle range, the results are much less impressive. If you stay poor for very long after being done with school, your gains tend to be miniscule.