The world's now cold, featureless, and culturally dead; nothing really new has been created since the Overlords came ... there's nothing left to struggle for, and there are too many distractions and entertainments.
This quote is from Chapter 15, where a pitchman tries to convince George Greggson and his wife Jean to join the New Athens colony. It is the strictest and most concise warning of what may await humanity if a utopia were ever to be created (unlikely to happen in the real world any time soon, without the deus ex machina of a benevolent alien invasion). Since Thomas More introduced the idea in the Enlightenment, many philosophers have rejected the notion of a utopian society as completely imaginary: an idealistic, abstract idea. Childhood's End gives the reader a glimpse into what such a society might look like, were it ever to exist—and the results are not encouraging. Clarke's conception of utopia might differ from a more modern one now. A modern conception of utopia would probably involve people working on renewable resources, such as farming, and everyone would probably live on, at least, a slightly lower level of modernity. But in the 1950s, Clarke's conception of utopia meant that everything was industrialized: food, clothing, shelter, etc. are produced in factories and available to anyone who wanted it. It is questionable whether the Overlords' utopia would have been sustainable without the Overlords' frequent supply ships bringing new resources to Earth.
In any event, the notion that artistic achievement would begin to decline in a utopia, while entertainment becomes increasingly important, is probably a very accurate prediction. Something similar has happened in history before when a large number of people are relatively happy. Rome lost its sense of society in increasingly violent gladiatorial games. Even Americans, in the second half of the twentieth century—and particularly after the end of the Cold War—turned increasingly to entertainment as a way to get along with everyday life, which lacked the hardships and struggle of their ancestors. In its predictions, the utopia of Childhood's End has much to say to modern America.