It is definitely not easy to answer this simple question with a word or sentence. Perhaps the best and only answer I could think of is that "it would make perfect sense" for the imminent future of humanity in 21st century and beyond. However within current world order we are living in today, it is far from being applicable for one 'teeny-tiny' reason - it would require canceling of what is well rooted today. The political system of Nation-State governmental polity that in one form or another exist almost everywhere on the planet. To cease that from existence is one of those Sisyphean tasks that is almost unimaginable to achieve. In simple words, in order to make City-State the only governmental polity we would have to nullify countries and to erase borders from the maps. Not only that, it also means the politics and politicians would have to reduce its influence and their numbers significantly which is also a task comparable to the impossible efforts from the mythical Greek underworld.

Artistic City Planet by Hazza42*

If you ask me, Sisyphus, a cruel king of Corinth who was condemned forever to roll a huge stone up a hill in Hades would have better chance to place his boulder on the top than we to cancel all countries worldwide and give the cities well deserved independence. Nevertheless, let's see, in one mind experiment, if we hypothetically had today such social organisation of the world, how the life would be different. What would we gain and what would we loose in our way of living?

But before diving into more details about how planetary society could work with cities instead of countries perhaps we could check a little history. Contrary to what we might expect to learn about history of governmental polity and even though we are taking countries and borders for granted today, in the past sovereignty over a defined space wasn't strict at all. Countries as we know them started to be a social organisation around millennium before today but in the essence, before industrialization took place in early 19th century their existence and jurisdiction were much different. Before that point in history countries existed in realm of several big empires which were constantly colonizing smaller nations who on the other end were in constant fight over unclaimed land or neighboring territory. Big empires along with remnants of city-states from the renaissance and number of small principalities and provinces, many populated with people of same nationalities, were constructed with centers of power that were still in big fortified capital cities surrounding their royal palaces. During all those times before industrialization up to the middle of 19th century, borders were pretty loose term that travelers crossed without any checkpoints or passports of any kind.

'Duomo' Florence, portrayed in Assassin's Creed**

History also had a good record of City-States, especially during the Renaissance period and it worked extremely well. Many cities across the Italian Peninsula, Mediterranean and Adriatic seas preserved their independence and flourished in almost every aspect of humanity. Milan, Venice, Rome, Turin, Naples, Ragusa and my favorite Florence was well-known examples from the late middle-age prosperity, especially in art and science. Even today, the spirit of the old days still lives in the very core of many Italian cities and in latest years the main example were Venetian online independence and autonomy referendums. They were unofficial in nature with their results being fully non-binding but in a nutshell, many residents of Veneto were and still are annoyed for the city paying enormous taxes to the Nation-State every year while in return they always receive less than given. The story goes even further in the realm of power and business and there are 'showcases' everywhere. For example the city of Niš in Serbia, where I live, was recently forced to give away its airport to the Nation-State to govern following a poor business deal between the national government and the owner of the airport in national capital which, believe it or not, limits the number of passengers and therefore further development of smaller nearby airports.

These two examples are just a tiny point of the iceberg in political absurdity of the relations between nation states and their cities. In hypothetical planetary cancellation of Nation-States and country borders even more ridiculous and superfluous affairs would follow and ceased to exist over time. If we exclude nullifying preposterous and vague speeches we heard every now and again, like those spoken by almost all politicians on power throughout the globe that "our nation is the greatest nation" or "our devotion to defend our nation is out of question", the real benefit would be in cancellation of enormous taxes for military budgets. No doubt, armed forces would not cease to exist but their move from national government hats into city halls would radically reduce the so called GFP or Global Firepower which in almost all countries took up to three quarters of national budget every year. If you are eager to know how big that numbers actually are I did some research in one of previous posts related to military named "Warfare Then and Now". I am not going into any debate how future warfare would look like in the world without borders but even if we found some logic in, for example, constant presence of Russian and American military forces in foreign countries such as Syria in recent years, it would be very far from common sense that in hypothetical City-State world, for example, cities of Seattle or Vladivostok would find any reason to send their armed forces onto Damascus streets. I really think that the most of the cities of the future would start minding their own business for the benefit of all the others. At least when GFP numbers are in question.

Earth’s city lights, Visible Earth catalog***

Furthermore, the potential of City-States networking outside of national borders is something countries cannot provide. The social policy within city walls could become more powerful and more effective. The big cities could also take care of their smaller neighbors more efficiently favoring production of domestic goods instead of hazardous imports of, for example, food over long distances. Traveling without unnecessary passport controls would be amazingly enjoyable and easy with technology providing invisible security controls on more checkpoints than just on airports, harbors or railway and road tolls without custom control at all. Yes, we would probably loose the joy of national competitions in sports but this would not be the real disadvantage, I am sure in one way or another the sport would survive and club competitions would take over with possible slightly modified Olympic games.

21st century and its further globalization primarily in the face of almighty internet is projecting completely new social environment for the future. Internet control is a battle that cannot be won even by using sophisticated surveillance or simple pulling the plugs out. Nor by introducing any national laws made in order to shape data control over the network. Similarly, according to UN, more than 50% of entire world population live now in cities and global migration toward urban settlements continues inevitably toward 60% at the end of next decade. For this trend being unstoppable as it seems, the network of cities of the future is something that will have the power to overcome the old ways. The outcome of the future countries that would become inefficient is inevitable and possibly even dangerous for further evolution of humanity and its survival. On the other hand, in a short conclusion, the City-State of the future could bring nothing less than another Renaissance to the world with new Age of Enlightenment with power of connecting people on a variety of brand new levels.

Image refs:
https://hazza42.deviantart.com/art/City-Planet-267257518
** http://assassinscreed.wikia.com/wiki/File:AC2_florence_concept.jpg
*** https://visibleearth.nasa.gov/view.php?id=55167

Refs:
https://en.wikipedia.org/wiki/Venetian_independence_referendum,_2014
http://www.intellinews.com/serbian-government-takes-over-nis-airport-139387/
https://en.wikipedia.org/wiki/Renaissance
https://aeon.co/essays/the-end-of-a-world-of-nation-states-may-be-upon-us
https://www.weforum.org/agenda/2017/06/as-nation-states-falter-cities-are-stepping-up/
https://en.wikipedia.org/wiki/City-state
https://www.history.com/topics/enlightenment
https://www.weforum.org/agenda/2016/10/forget-the-nation-state
http://www.un.org/en/development/desa/population/publications/pdf/urbanization/