There's an extreme amount of assumption baked into that.
1. Genetics is the only factor that determines merit for wealth?
2. Genetics merits extreme differences in income/wealth?
2. We're talking about social mobility, not the level of inequality.
1) One does not have to be intelligent to be wealthy.
2) Those born wealthy do not have to be intelligent to remain wealthy - they just need not be a consumeristic imbecile.
3) Genetics has such a small impact, if any, on social mobility - quite the opposite. If I am born a Kennedy I am more likely to never fall below my birthright social class because of my family lineage.
4) You contradict yourself. You say the nobility rules by force, then immediately follow that with "If that society becomes free" - how exactly would that society magically free?
2. 70% of rich families lose their wealth by the second generation, 90% by the third. [A]
3. IQ is a greater predictor of income than parental SES. Someone in the 95th percentile for IQ has a higher EV for earnings than someone born into the 95th percentile for wealth.
4. A number of revolutions have happened. If you don't believe that modern France or Japan are freer than their feudal predecessors then I agree that we are probably not on the same page.
I've seen the statistic quoted in so many places, yet have never found a link to the actually study. No info on sample size; we don't even have a definition of 'wealth' used in the study.
[1] https://www.wsj.com/articles/SB10001424127887324662404578334...
Let's say we've got a feudal society with dumb nobility that rules over a smart peasantry by force. If that society becomes free, we'll expect smart peasants to move up and dumb noblemen to move down across the first few generations... but once they've moved, they've moved. Ergo, declining social mobility.