Barring a setting conceit to the contrary, anything PCs can do, the NPCs should also be able to do. If the PCs are not special in some way, then the NPCs should die at roughly the same rate the PCs do.
The conceit is that the PC's are particularly unusual in several regards, through granted there is nothing that prevents NPCs from success on their own.
First, PC's are built on a 32 point buy. Average NPCs really are around 15 points, and my average is NOT actually 'the lowest stats you'll ever see on any NPC ever' or 'only the bottom 1% of NPCs'. Important NPCs are generally built with 21 points. Even as much as the elite array is really rare.
Second, the PC's are 'advantaged' characters that start with a bonus trait. The majority of NPCs aren't advantage, though obviously, someone like the Crown Prince has the equivalent of considerable starting advantages in life - more even than the PCs had.
Third, PC's are 'destined' characters that start with destiny points that reflect unusual luck, divine favor, special destiny, or other supernatural connection to the world. Probably not 1 in 10,000 NPCs is also destined.
Fourth, and perhaps most importantly, the 'story' begins in a situation where a number of these '1 in a million' type individuals happen to have converged at the same time (by chance or design of the gods).
Almost no NPC in my game world is so advantaged. Even the elite stat array is rare, and only the BBEG in my campaign of all the other NPCs thus far encountered has a 32 point buy and I strongly avoid the trope seen so often in 1e of NPCs with vastly better stats than would be available to PCs. Indeed, the BBEG is one of only a handful of NPCs that they've met that also has destiny points. In short, the PC's, while still being basically the same sort of thing that NPCs are and playing by the same rules, have a significant measurable advantage over all or most of them. If they didn't have these advantages, they almost certainly couldn't be the sort of heroes that they are and taking and often surviving the sort of trials and challenges that they face.
Do you run a game so lethal that PCs rarely survive over 5th level? If not, that's where you're effectively sweeping the issue under the rug - allowing the PCs to survive, but assuming most of the NPCs do not.
Even with all these advantages, the game is sufficiently lethal that hardly any PC's have survived to 7th level. Most of the PC's have died, much less the NPCs. Partly that's the inexperience of my players - all but one of the deaths have occurred as a result of the player's splitting the party. The game isn't designed to be as lethal as it has been. The game is designed to create the story of a group of exceptional heroes who will make an indelible mark on history and later be remembered in song. Obviously, the PC's aren't the only ones to have done so - the PC's have heard songs about past glories as well and come across the evidence of mighty past deeds. But the PC's are a bit special.
Yes. But, as above - if the PCs can survive, so can the NPCs. And certainly, it isn't like threats only *just* started popping up, right? There have been loads of threats for centuries. So in the past of the world there have been people doing the same things the PCs do. And so, those people should have risen in level. If there are thousands of threats, all the time, there then are and have been thousands and thousands of people earning XP working against those threats. Where are they?
All over the place. I listed some examples of NPCs that 'made good' in an earlier post. It's not as if I suggested that there are no higher level NPC's. But as the level increases, they become increasingly and exponentially rarer. Part of that is that effective challenges become rarer. Part of that is the more challenges they face, the more likely it is that they died. Part of that is that most people aren't actively seeking out difficult challenges and 'retire' from a life of danger more or less as soon as they are able. Part of that is that most people become old some time around the time they become experienced. There have indeed been NPCs of 20th level or higher in the world. At any time, there might be a half-dozen such NPCs. Over the course of 5000 years, there have been a considerable number of them.
I guarantee the PCs arc will largely emulate the arc of NPCs by past experience. Those that obtain high level will tend to have the same pressures acting on them. Most of my campaigns wind down before 15th level with PC's in a position of leadership, wealth and relative security and players that are very hesitant to take unnecessary risks with their now treasured and largely irreplaceable character.
Well, actually, it kind of is. If you aren't playing E6 or the like, the personal power levels characters and creatures can reach in RPGs is vastly above that attainable in the real world, and the methods of gaining that power (XP awards) does not exist in the real world. The situation in the fantasy world is thus inherently ridiculous, from the real-world point of view.
From the real world point of view I suppose, but it's not necessary for a fantasy world to be fully consistent with the real world - if it was, it wouldn't be fantasy. All that is necessary is for a fantasy world to be consistent with itself. FR fails to be consistent with itself for the core 'zero to hero' narrative that is iconic D&D. This is easily demonstrated with concrete examples from published FR modules. The stories set in FR always have as their central protagonists NPCs that do the heavy lifting and reap the main rewards, while even the greatest PC heroes are treated shabbily and as of little account. The FR are always about the NPCs who possess advantages and abilities the PCs can never match. None of the stories set in the FR necessarily make sense unless you assume all the high level characters in the game are jerks that are also not self-interested and also stupid despite their high stated intelligence.
For example, Gygaxian demographics provided that humans and orcs or humans and hobgoblins presented a reasonable challenge for each other, so that human communities with their 0 level fighters and 1st level fighter leaders were reasonably threatened by 1HD and 1+1 HD invaders. But when FR came out, one of the problems my play group at the time spotted initially is that FR military demographics assumed common soldiery were as potent as the most elite militaries of other settings and yet initially the demographics around orcs or hobgoblins didn't change and the narratives around threats of demi-human invasion didn't change despite. The immediate question this raised for us was, "How is that the ugly demi-humans aren't extinct?" Even with Gygaxian demographics, the number of orc tribes a 10th level party could simultaneously face and eliminate made the question of whether orcs should be a real threat somewhat interesting. With FR realms demographics, the whole thing got ridiculous. Those Caves of Chaos were barely a threat to the Keep on the Borderlands as it was. In the FR, why doesn't the butcher, the baker, and the candlestick maker just clean it out and take the gold for themselves considering how little it would trouble them? The normal D&D path from zero to hero doesn't seem to be there if you start at 1st level. Whatever opportunities that 1st level characters have seem highly contrived given that the bountiful wealth they reap could be reaped far easier and at far less cost by any half-dozen NPCs. To jump into the story requires starting at 3rd or 5th level so that at least you aren't a below average individual and are of some use, and even then if you stick to the canon, the best this provides for you is the opportunity to have a good vantage point on the awesome cut scenes. And I mean that later point literally, as its not unusual for a FR module - even and especially the early ones - to have the direction to the DM, "The PCs can do nothing to change the outcome of this scene."
