That seems like an odd premise. Faerun has always been on the extreme high end of high fantasy, as far as D&D settings go. There's more going on in Faerun, with more high-level spellcasters, than every other published setting put together.
Designing for the default D&D world isn't an odd premise within the usual definition of "
odd"
Even anecdotally, when people talk about their home settings, most make note of how it's less fantastic than the Forgotten Realms. I can't recall anyone ever mentioning that they play in world where high-level adventurers are more common than they are there.
It's true that people's experiences and tastes differ on these things. The most common other rule I've heard is 1/100 at level 1, halving per level upward. That rule produces greater numbers of character-class NPCs at low levels, but fewer or none at higher. As an algorithm I feel like is good because it is easy to get your head around, but bad because for
iterations = levels it doesn't extend properly upwards. This is in truth a complex discussion and I don't want to over-simplify. Some elements are whether we play much at high level and use high level NPCs to challenge our parties? Do we assume organisations such as monasteries, holy orders, clergies for multiple gods, arcane colleges, or thieves guilds? Have we thought about how many character-class equivalent NPCs are needed to populate those (or is our 5th level Rogue the top of the tree? and if so, what happens after that?) What is the base population of our world? (I believe Faerun has 70m.) Often people hand-wave "
rare"... is 1 in a million rare? Or do they mean something else? If their world has 3 million people, maybe 3 epic tier NPCs turns out to be right.
That all said,
we clearly hear different anecdotes! Anecdotally what people tell me about their campaigns maps pretty well to what I have proposed. But this is the wrong sort of assertion to make, isn't it? Very he-said, she-said with no facts on other side.
It would probably be more accurate to say that many DMs haven't felt the need to map out a distribution in detail. There is a contingent chiming in with "
I always hand wave it". It's impossible to say what that implies in terms of densities unless they will also tell us their world populations, their party levels, how many NPCs their party has encountered all-told, and how many were character-class-equivalents. Clues of that sort. I suspect we will find an extrapolated density for their world turns out close to that proposed.