I think your wrong about the "all time high social mobility" my understanding is that the peak was reach 40 years ago.
I'm currently reading Thomas Piketty on the subject
40 years ago, most of Eastern Europe was under North Korea-like Communism, China was still enforcing the one-child policy, and South Africa was still under Apartheid. Not to mention the looming fear of nuclear annihilation. I’m not sure how 40 years ago, we were better off.
But I’ll look into Thomas Piketty.