I won't argue the fact that Cyrus was enormously influential to the course of fertile crescent civilizations. That said, really he was just one of the first Fertile Crescent leaders to see not only the weaknesses of the city-state, but the means by which to exploit those weaknesses. (It's just that what he did to exploit those weaknesses was so radically orthogonal to the way things were done at the time that even today we're still calling the guy Cyrus the Great.)

Once the Nation-State idea was out of the bag, city-states just couldn't compete. Even the Greek city states fell to Macedon because by the end of the Theban hegemony, they were all so weak that even Macedon, effectively a baby nation-state at the time, was able to take them out. City-states wouldn't be able to compete today either. You'd always have to have some great power around to back them. Even Federations of city-states have faired poorly historically.

It's like continuing to rely on bronze swords once people got the hang of making iron ones. It no longer mattered all of the reasons that bronze was better for you, because iron was better when it counted. The cat was out of the bag, you could try to put it back in the bag, but everyone's already seen it.