I think this is just an "r>g" rule made famous by Piketty. When r (return on investment) is bigger than g (overall economy's growth), the rich get richer. It's unsustainable in a pure mathematical sense, the rich cannot own more than 100% of economy, so at some moment this system breaks, and one can only hope that it doesn't break violently.

Piketty somehow wrote a 1000 page book about that, and a slightly controversial one, but an idea is very simple and, I think, hard to argue against.