Is Capitalism Working?

Next Big Future has a summary of the theories of Thomas Piketty, who claims that unfettered capitalism causes highly unequal societies as well as a graph to demonstrate his theories. I am dubious.

According to Piketty's theories (and he apparently has data to back this up), a system in which investment returns exceed growth rates produces more inequality. The data in the graph also show that growth rates exceeded investment returns for the past century. Piketty then combined this actual data with assumptions that growth rates would decline and investment returns rise. Both of these assumptions are uncertain.

The really weird part is this is completely opposed to Karl Marx's assumption that competition would drive investment returns to zero.