I fully expected a dodge when I wrote that and wasn't disappointed.
So, in other words, you aren't interested in understanding why other people have problems.
Poweful NPCs, mentioned or not, are relevant to the idea that they somehow make the PCs feel inferior.
That sentence is silly. You can't claim something relevant to the idea, and by using the word "somehow" say you don't know what's relevant to the idea.
The issues is some players want to play in a world where Joe the Fighter is the hero, where his name will be the one used in stories that reverberate through history. In Forgotten Realms, the feeling is that Elminster and Drizzt are the characters that the stories are written about, and the player's characters will be no one even in the fictional world, that anything they did could and would have been done by the NPCs.
they exist in both worlds unless you run a wholly unbelievable game.
I had to check; yes, yes we are in the D&D forum. The NPCs leveling at the same rate as PCs is wholly unbelievable, and barring that, E6 is more believable than 35th level NPCs running around.
18th was still enough to overshadow the War of the Lance PCs for most or all of the modules(can't remember what level the PCs finish at).
Yeah, overemphasis on level probably misses the point. As I said, the high-level good NPCs in DLA had notes that they weren't going to go out and adventure. The more activist NPCs are a problem that static NPCs aren't. There was no one in the setting that was going to take over for the War of the Lance PCs.
The vast majority of games don't even reach 10th level from all the polling released over the years. That makes the even the 15th and 18th level NPCs in the various settings one that "overshadow" the PCs.
I'd say the people who have this concern either reach higher levels or think about reaching higher levels. There's a big difference between a couple 15-20th level NPCs in a setting and more than a dozen 20-35 level NPCs in a setting. In Unther, the FRCS lists NPCs of level 30, 18, 16, and 15. If you want the PCs to oppose the Mulhorandi invasion (which I have no idea how it's happening, given the most powerful mentioned NPC in Mulhorand is 11th level), they're going to be secondary to those NPCs until they reach high level. Whereas in most settings, a small nation like Unther is not going to have a single character of 15th level, and it's likely if they do, they won't be able to get involved in adventuring. And the gap between a 10th and a 15th level character is a lot smaller than a 20th level and 35th level character.
And part of it isn't strictly about levels; it matters that you open the FRCS and the first real content is Elminister, and that the text is regularly interrupted by a half-page devoted to an NPC. It's saying that these are the characters who are important in the setting. I can't name any NPCs in Golarion or Eberron or Planescape (except the Lady of Pain) or Spelljammer, but you mention Forgotten Realms, and Drizzt and Elminister come up.