I think that the "nation states" are losing their viability in today's world as a political form.
In the beginning humans, like any other social mammals, were organized in tribes. When the civilizations evolved, the need of a higher political organization appeared : the nation-state. The primitive states were autocratic monarchies that were usually created when a powerful tribe instated its dominance over the others on a certain territory. In order for this states assure their legitimacy and perennity, they had to ensure the sociocultural homogeneity over the controlled territory.
The main advantage of this superior organization was that created the sens of unity and because of this a large number of people started to work together and to collaborate for achieving a common goal. Inter tribal wars inside a "nation" became a lot less common and laws, protected by the powerful king, were instated. This crated order and stability and as a result humanity evolved greatly. Wars against the "outsiders" were, on the other hand still acceptable and even considered necessary and moral. As a result, in a chain reaction, all the other sociology-cultural groups had to centralize their power in order to ensure their survival.
This world of "nation-states" exists already for more then 5 millenniums and it became a part of human subconscious. That's why for most people a different form of governance seams impossible.
But what is the situation today?
The humanity evolved to a stage where the life of a human been if lived at a planetary scale. Physical travel around the globe, became a matter of hours and information transmission a matter of seconds.
We are technological advanced enough to control and use a major part of our planet energy. We have become numerous enough to be able to deplete our planets finite resources, and the population is still growing at an alarming rate. We have the weapons capable to wipe out the whole life on earth if we are crazy enough to start a all-out war now.
No political decision really makes sens right now if it does not consider the global context. International organizations (UN, UNESCO, etc.) exist right now but they don't have enough power to really set a direction.
But lets remember what the nation- state is. A political structure created to centralize the power, to represent and control a sociocultural homogenous group. As you can see it completely lacks the global perspective in its very definition, and also the sociocultural homogeneity inside of a nations today no longer exists. A nation-state is unsustainable if people do not feel "represented" by their leaders and do not identify themselves with the direction set by the central power.