I'm not trained in this field and I have not yet read Piketty's book, so take this all with a grain of salt, but my understanding from the discussion I have rea

I'm not trained in this field and I have not yet read Piketty's book, so take this all with a grain of salt, but my understanding from the discussion I have read is that Piketty's claim is that, if the rich can multiply their wealth faster than the middle class can become rich, then wealth becomes a hereditary entitlement rather than an incentive for hard work, and the meritocratic aspect of a capitalist system breaks down. The Marxist thesis is essentially that the bourgeoisie (the rich) exploit the bourgeoisie because they generate more value from the proletariat's work than they pay in wages. Even if one accepts this line of reasoning, it is sort of orthogonal to Piketty's argument. One could imagine a society where the bourgeoisie exploit the proletariat, but, with intelligence, vision, hard work, whatever, the workers can still become members of the bourgeoisie, i.e., the "American dream." Piketty says that, under certain economic conditions, even this becomes impossible.