> He addresses this line of logic
He doesn't, and it's not a line of logic, it's an interpretation of the evidence. Graham didn't even look at the evidence, and came to a conclusion that becomes very hard to support once you've had a glance at a chart of the available evidence (i.e. deep analysis isn't required to recognize that his proposed mechanism is flawed).
Also, you should really heed my 3rd note. You cannot speak effectively on income inequality without having read Picketty. It is by far the most significant treatment on the subject. It goes far beyond my simple counterpuntal analysis here to bat down a poorly thought out and completely unresearched pure logical argument. At least read a couple informed reviews of Picketty. I seem to recall FT, the Economists, Mankiew and Krugman having worthwhile, messy and conflicting thoughts after it was published. Graham just is not a useful resource on income inequality.
To address your point: Yes, Macro 101 makes it clear that trade multiplies wealth, thus wealth is non-static. The Graham argument you cite is incredibly elementary, and he is not the source to be citing on the topic. That particular question is miles more elementary than any discussion of the impacts of income inequality on macroeconomic factors. Bringing it up when talking about Income Inequality is kind of like citing a clunky explanation of Newton's 2nd law of motion in a discussion about time dilation effects of supersonic flight (which relies on non-Newtonian physics). To wit, analysis of Income inequality requires going beyond the overly simplistic macroeconomic models that have led Economists to mispredict the effects of income inequality for so long. Macro 101 is not useful here. The current evidence in the field is conter-intuitive and difficult to reconcile for expert economists.
Also, again, Graham's essay is very outdated and has, to be charitable, been surpassed by more careful and thorough resources on the topic.