Historically large differences in relative wealth (gini coefficients) are strongly correlelated with the breakdown of society and revolutions etc. The US is already pretty high in OECD terms, and has pretty appalling stats in access to health care incarceration rates, crime, educational outcomes for poor kids etc that tend to go along with having a high gini coefficient. You have a huge and growing pool of americans who have little to no real hope of being able to get their kids into even the middle class.
The Trust Fund Kids of the Walton family (Walmart) have a net worth equal to the poorest 90 million US Americans. (That absolutely staggers me).
Now assume for a moment that they have little or no interest, aptitude, or industriousness in using that capital in an efficient manner to grow further enterprises (ie improve the country) but simply want to live as emperors - I don't know if that is actually true, but in some cases it may be. Such colossally rich individuals who have not created that wealth themselves but have simply chosen their parents carefully are in effect a useless nouveau aristocracy.
That equity would be far better in the hands of hard working people further down the chain who could use it to start businesses, improve infrastructure, develop technology, educate their kids etc, but given the size of their money pile and the relatively small birth rates of today, there is a strong possiblity that their dynasty could maintain or even with minimal effort grow their total holdings as a proportion of GDP over the coming decades/centuries.
I have no problem with entrepreneurs getting rich, but that wealth needs to be returned to the rest society within a couple of generations if you want to have a dynamic society based upon egality, fraternity and liberty (that is what the French revolution was all about) and not a world of gated communities and violent ghettos. How you go about constructing a society that achieves that is left as an exercise for the reader, but massive inheritance taxes (above some level) seem like a pretty useful component.