> The pearl has just begun to be produced. Do you think that the kids of the current top billionaires will not be at least middling billionaires? ... The US has lower social mobility than most European countries. Considering the fact that Europe is part of the "Old World", that's a worrying fact in my opinion.

I'm not sure that this is actually true, wrt. "old" wealth. Mostly, because Europe's high income taxes and broadly business-hostile environment makes it very hard for new wealth to compete with old wealth, and especially unattractive for "old" wealth to get involved in riskier, more innovative ventures of their own as opposed to just kicking the can down the generational road.