I don't agree with this - if only because it has the implication that I can't run a satisfying high-level campaign unless I put a lot of work into world building, which as a GM I just don't have the time/inclination to do (and I suspect I'm not alone).The failure is actually in setting/world building.
I want to say more about this below.
I think this is more or less the right way to go, but as well as stating it in the sort of metagame language you use, I think it's also helpful to try to give it an ingame logic as well.My own answers to all these questions and concerns can be summed up in just two words: "narrative" and "solipsism". The demons in question have not already overthrown the local regime because the heroes are entering the story at just the right time and should be able to prevent them from doing so; it is all part of the narrative.
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There is no threat other than the current one. Only the current narrative counts. There are no heroes other than the ones in the party (okay and some other story-relevant NPCs).
Dropping this assumption helps give the solipsism an ingame logic. If the PCs are the only heroes, then there is no other high-level adventure taking place in the gameworld besides theirs. (This can be tricky if PC death is frequent, but in high level D&D raise dead/ressurection shouldn't be that hard to come by. If you need to introduce a new high-level PC, handwaving it shouldn't be that hard.)Assume for a moment that the campaign world is populated by dozens of adventurer types, if not hundreds (and the mere fact that the local tavern always seems to have two or three standing by whenever a party member gets himself or herself killed seems to suggest this is the case!).
In my last high level game, the demonic threat emerged more-or-less contemporaneously with the PCs becoming ready to meet it. And it was a threat that the gods couldn't deal with, precisely because it was the result of contracts the gods had entered into as part of the original settlement between the heavens and the hells. Events in mid-level adventures set up some of the basic ideas of these constraints on the gods, and then the ramifications were introduced as challenges for the PCs once they became high level.characters are called upon to deal with threats and menaces that are so powerful that it is hard to understand why they have not already overrun the local area, the nation, the world. You can see his point. A party of 18th-level characters (not even particularly high-level, see; not these days) might be called upon to deal with an incursion of demons or what-have-you. Those demons are demonstrably powerful (excuse the pun) and it does bring us to ask why they have not already overthrown the local regime and taken control of everything under the sun.
In 4e, the ideas of the Dawn War, the Compact of Heaven, Primordials wanting to reassert their power, and the Primal Spirits keeping the world safe, all create ways to plausibly have threats emerge that only epic heroes can deal with, but which emerge only when you want to present thsoe challenges to the PCs. (The Plane Above also introduces the idea of travelling back in time to change mythic history - this is the perfect excuse for epic PCs to find high level opponents who aren't just hanging about failing to destroy the world.)
Going back to the world building thing - I think that developing an ingame logic that supports narrative and solipsistic solutions to the problem isn't about world building at all. It's about scenario design. In particular, the resolution of low- and mid-level adventures should always be opening up the possibilities and prospects of your gameworld, with suggestions that there is more to the machinations of the gods, the devils, the efreets, the lich lords, etc etc, than they have resolved in this current adventure. You don't need to give details, just hints and a general "vibe". Then when you get to high levels you start cashing this stuff out. You can make up the details at that point, as long as they aren't grossly inconsistent with what came out before (in my experience you'll generally get away with minor inconstistences unless your players keep very detailed notes).
That's how I like to do it, anyway.