If they don’t have to work to live, they’re no longer working class (Musk still works at Tesla, but clearly doesn’t need to). But I’m not sure what your point is. Are you saying that those who derive their wealth from the labor of workers but who can’t afford a yacht should be treated differently than billionaires? I’m not arguing that we shouldn’t have progressive tax rates, I’m only saying that an arbitrarily defined ‘middle class’ exists solely so that you and I are distracted by exactly these discussions, and provides no benefit to determining what is justifiable economic policy.