The Piketty book is often compared to “Das Kapital,” but that isn’t really accurate. Piketty doesn’t believe in anything as woolly and mystical as dialectical materialism. He doesn’t say that mankind is headed for a classless nirvana. He simply generates data which indicate that, under normal conditions, the rate of return on the investments typically held by the wealthy has exceeded the percentage increase in GDP in Western industrial nations. As a result, barring significant policy changes or disasters (wars, depressions, etc.), the trend will be for the affluent to possess an ever-increasing percentage of national wealth.
You could call it something like the iron law of oligarchy, but, as noted above, it can be reversed. Why do I bring this up? Because Bernie Sanders represents Marx in this debate, and Elizabeth Warren stands with Piketty. At some point in time during the campaign, you have to think there will be at least a bit of a debate between the two of them on this point.
If it happens, expect Warren to win it in a landslide–partly because she is the more intelligent of the two candidates, but partly because her position is much easier to defend.