I would agree with the gist of your comment, which is that autarchy can be a stable equilibrium.
However, I don't read Piketty as saying that inequality itself is unsustainable. Rather, inequality tends to undermine democracy. Thus, "unsustainable" here should be read to mean "unsustainable in conjunction with our commonly held egalitarian, democratic values."
However, I don't read Piketty as saying that inequality itself is unsustainable. Rather, inequality tends to undermine democracy. Thus, "unsustainable" here should be read to mean "unsustainable in conjunction with our commonly held egalitarian, democratic values."