Curious, what exactly stops these rich families from simply moving their wealth off shore? I'm sure there are many other countries that would bend over backwards to have these uber rich families living in their country instead of the USA.
They would have to give up their US citizenship. Financial implications aside, few are willing to go that far. The financial implications are also large. You have to pay an exit tax, which would be substantial. And giving up US citizenship to avoid taxes is illegal, so there's that too...