The previous post is right, I think.
Points of Light is not a setting. Rather, it is a anti-setting. It's a description of a game run without a map with all the blanks filled in, without a history except in the most general sense, without politics or sourcebooks. And, a setting can easily remain as points of light for a while, but sooner or later, both players and the DM are going to want some consistency, so the DM will figure out what king just sent the tax collector and what the tax collector said the taxes are for (the king's war against the ____ or reparations to ____ because of the treaty with ______). The players will ask the traveling gnome merchant what settlements lie ahead on their trip to the mountain fortress of the ancient witch queen. The DM will want the players to make some difficult choices about what side to take or whether or not to leave an evil in place because destroying it would have worse consequences than the evil has itself. And when those things happen, little by little, the anti-setting that started out as PoL has evolved into a more concrete setting where the DM at least knows where there are points of light, where there are pools of darkness, and where, if anywhere, there is a clean and well lit place.
There is also another way to look at the question, but it also goes to the heart of the issue. Points of light (used another way) is not a setting. Rather, it is a state of a setting. The world would not always have been describe as points of light. It may not be described as points of light for too much longer. Points of light just happen to be the stage of a setting that has some interesting and classic adventure opportunities.
What does that mean? Take some of the people groups of South America as an example. The Auca, for instance. As late as the mid 20th century, they lived in small groups scattered throughout the jungle with stone-age technology. They often had hostile encounters with neighbors who might be at similar levels of organization and technology (other people groups and other tribes) or who might be much more organized and advanced (people from the modern societies existing outside the jungle). They were in a similar position to a small group of people in a points of light style or stage of a setting. Just substitute gnolls for the other tribes and other indigenous people groups and higher level monsters for the civilized people (having a gun makes you a higher level monster because it's an at will power rather than a 3/encounter power like a javalin) and you've got it. The important thing to realize about the Auca, however, is that they were a dying people. They got by. Lots of them lived to adulthood without getting killed in a blood feud, starving to death, or being shot. But each generation, there were fewer and fewer. It is reasonable to speculate that, had things continued in the same points of light manner, there might not be any Auca left alive today. As it happened, they were encountered by missionaries and changed a number of their ancestral ways of life. From what I can tell, their existence is no longer analogous to the small villages in the points of light setting.
That was inevitable. Points of light is not a permanent state. Before there were points of light there was probably some pretty good lighting. (In the 4th edition default setting, there were dragonborn and tiefling empires and a number of human kingdoms). After it, the points of light will either grow or go out. You could end up with Greyhawk, the Forgotten Realms, or you could end up with Athas or Midnight. But it won't stay the same.
To continue the previous analogy with a few movies, the points of light could end like The Mission. It could end like Apocalypto. Or it could end like The End of the Spear. But by the end of the movie, it won't be points of light any more.