And I think I addressed this earlier in my 5 campaigns post.
In the first campaign, the PC's carve out a nice little empire. However, they will not pacify an entire WORLD. There just isn't enough time for that. They die of old age before they can do that. So, they build a nice empire, say the size of China.
However, even within that empire, there's still PoL. It might be brighter than outside the empire, but, there's still some shady places. So, our second generation goes and deals with the little stuff that got left behind the first time around and secures the empire. Once they move on to Epic levels, they extend the borders of the empire. They now control an entire continent.
Then the 3rd campaign starts. Taking a page from the excellent ideas of Steven Erikson's Malazan series, our empire is now challenged from without. A neighbouring empire of demon worshipping fanatics decends upon our fledgeling empire with a vengeance. The PC's start out by battling first the leading edge of the invaders, then working their way up the food chain, they stop the invasion and push them back. Empire is restored.
Well, there's 3 nice solid campaigns. What else do you want from a setting?
Or, you can go the other way and say that every time your PC's hit those kinds of levels, you go to a new setting. Fantastic too. I'm all for that.
Either way, you're golden.