The danger of inequality is not something new, and the problem isn't that someone is super-rich, the problem is that median purchasing power is going down, because economic growth is not going into the hands of the lower and middle class, and this is causing demand to fall in the market, leading to an inevitable economic downturn. The dangers of this were discovered a century ago during the Great Depression, and are well understood and not really considered controversial anymore: https://en.wikipedia.org/wiki/Keynesian_economics