Thomas Piketty is being criticized by the right not mainly because his historical and economic analysis is wrong (even The Economist accepts that it is mostly right), but because his main policy recipe (a progressive global welath tax) is allegedly infeasible. Even the French economist himself is hesitant to defend his alternative. In the book, he says that this proposal is utopian (although a European version would be feasible, he argues), and in the CUNY debate he started by saying that the fourth part of the book (containing his proposal) is more speculative than the others. Ultimately, the issue boils down to determining whether world federalism is a practical proposition or not. Actually, we are running in that direction, and we have been running towards it since politicians, thinkers and scientists such as Albert Einstein, Winston Churchill, Bertrand Russell and Mahatma Gandhi advocated the idea of world federalism in the early Twentieth Century.
World federalism, or world government, is the concept of a political
body that would make, interpret and enforce international law. Inherent
to the concept of a world government is the idea that nations would be
required to pool sovereignty over some areas. In effect, a world
government would add another level of administration above the existing
national governments or provide coordination over areas national
governments are not capable of adequately addressing as independent
The most relevant model for the incremental establishment of world
federalism may be the European Union, which politically unites a
wide-ranging group of nations, some formerly hostile, over a large
geographical area. Though the EU is still evolving, it already has many
attributes of a federal government, such as open internal borders, a
directly elected parliament, a court system, an official currency (Euro)
and a centralized economic policy.
The EU’s lead is being followed by the African Union, the Union of
South American Nations, the Organization of Central American States, and
the Association of Southeast Asian Nations. A multitude of regional
associations, aggregating most nations of the world, are at different
stages of development towards a growing extent of economic, and
sometimes political, integration.
Now more than 40% of the population is in self-proclaimed federal states. These states, the European Union and other unions should and can lead the world towards global federalism. Even China will have to join if it becomes a democracy, according to Branko Milanovic in "The Haves and the Have-nots", and then they will have to read again about the ideas of the great Chinese federalist Chen Jiongming.
To me, the fourth part of Piketty is as good as the others.