Of course there are reasons to be skeptical whether or not the traditional nation-state, at least the structure we've had for the past 200 years or so, will be with us by the end of the 21st century.
Climate change, resource scarcity, population increase, economic decay, superstition and ignorance, along with the growing influence of non-state actors across the planet are likely going to place huge strains on all nations, even the most powerful. The question of “legitimacy” could very well gather momentum among more and more people. But what could replace the traditional state?
The short answer is that we don't know at this point, but resilient communities, transition towns, tribal networks were, until quite recently, ideas espoused by only a handful of people, often on the fringe of society, but which is now no longer the case. The transition movement, for example is spreading across the globe. In the U.S. go to http://www.transitionus.org/ Transition United States.
John Robb of Global Guerrillas, who has written a lot on asymmetrical warfare and “system disruption,” has a couple of interesting pieces on resiliency. See
http://globalguerrillas.typepad.com/globalguerrillas/2010/01/why-a-resilient-community-network.html Why a Resilient Community Network? As well read
http://globalguerrillas.typepad.com/globalguerrillas/2010/01/journal-tribal-opportunity-space.html Journal: Tribal Opportunity Space.
As we slowly get beyond abstract ideas and put together specific proposals and detailed plans for a particular community or region, we will likely see a widespread interest in the development of alternative “states” of various types.