"We're not arguing about absolute social mobility of a society. But the relative social mobility within a society. Nor is anyone advocating Cambodia. Nor has anyone denied that generation to generation most Americans are wealthier than their parents."
I think you're being confused, the overall mobility is merely the sum of individual experiences.
BTW
Drug legalization folks cite Portugal as the example of a country that legalized drugs and saw the percent of its population that uses, and will use, no matter what, steady.
At 17%.
If people CHOOSE to put themselves into the lowest 20% of society and KEEP themselves there, where's the unfairness?
Your arguments put it back on government services in childhood.
If most people denied that kind of goodies still stay level or move up, how's it unfair?
Especially since the approach to "level" society works by destroying alternatives and crippling opportunities.