Could we be missing the point just a little when we use the label 'post-apocalyptic' for a setting, or for a type of fiction like D&D? And missing it always, regardless of specifics?
Blood of Prokopius flicked the ponder switch here, referencing a post at Grognardia.
Unless the principles shaping a setting or fiction type prevent there being successive waves of identifiable development and decline, or eventually remove any perceptible legacy of the process, a world will always evolve until recognisable to some degree as post-something, whether the end of that something was actually apocalyptic or not.
If we're talking more about the mental atmosphere in a setting or associated with a type, it's natural that in moving into a future the occupants will look into the past, for things of value like knowledge. Assuming of course there's an awareness of a future at all.
It's also natural that as creators or participants we will project our approaches onto a thing, populating it with our interests and hang-ups, and that if they think like us, the occupants will presumably also comprehend the loss and be intrigued by it. Maybe a sense there is or has been something better, proof we can have it too - or can't.
It's possible to imagine 'post-creation' fiction, in which the thing that's past is only the very beginning, and even a time which wouldn't ever become myth, legend or history if conditions didn't let it develop, leave a record or survive. It's also possible to imagine a 'pseudo-post-apocalyptic' type in which a false record is created, which would add a twist to delving and discovering - if recognising the truth is allowed by the principles.
Going beyond that, what if the possibility of successive waves of development and decline itself were prevented by the overall framework? Could a thing with no possibility of change, no time, be attractive? How would we express and experience it?
Could it be that settings are 'post-apocalyptic' in the broad sense of 'after an ending' because there isn't much choice otherwise? Because if they aren't, they don't interest us? And because if they aren't instead simply 'post-creation', we can't know them?
Lots of questions. I feel like I'm overreaching a bit, and that I've overlooked something crucial. But I'm ready to be set right. We are fallible after all, and it seems we know it.