It ain't a demand. It's an in-character choice to advance along a particular path. The Wizard sees the Necromancer in action, and decides to pursue the dark arts (but use them for good).
Except that it takes years - many many years - of bargaining with the right creatures and selling a piece of your soul each time. It is not something one can learn overnight and powers already exist with such "flavor" in 4E. The Wizard can take those from the norm, but he can never master what took an NPC necromancer much of their (undead perhaps) life in a short period.
If you're letting PCs hang around for
years so this becomes an issue I can see.
The Fighter faces off against a gladiator in the sand-pits, and then has the gladiator teach him some nifty tricks.
There are already gladiator based feats, a theme and a build for fighter.
The Cleric witnesses a strange miracle from a rival of his gods' and starts asking for the same.
Gods are hard to fathom at the best of time so if he does anything that is completely up to the DMs whim. It could even be a core part of the story why one God may interact while another cannot.
The Thief sees a rogue from another guild swiping the treasure out from under her, and says "I want to do that!"
Then the PC just needs to get better at it.
None of these don't have good explanations for why the PCs couldn't do them easily.
It ruins the mood and feels unfair to say "Oh, they did that with skill and ability and magic and suchlike that you have no hope of ever acquiring, so sad, but hey, it's mysterious and awesome, amirite? Aren't you mystified? Don't you feel awed?"
I've honestly never had this problem once in 4E.
3rd edition? Consistently. 4E? Never.
This happens because, presumably, all mortals are made more-or-less equal.
I am never going to accept this premise. If your argument relies on it, every single thing is going to collapse like a house of cards because this statement
isn't even true in real life. In a world of fantasy where PCs and other creatures routinely do things that are beyond human in our world, it's going to be even MORE incorrect.
Any given human has the ability to go out and do something that any other human has the ability to go out and do
Absolutely 100% wrong. On every single level. For one:
1) Some of those things took an immense amount of time to learn, either due to requiring rare materials or certain study.
2) In D&D, some of those things that NPCs sacrificed or plain did things PCs wouldn't be willing to do (either practically or because it's just too horrible).
But okay, let's make a bet on avatars. You go out and in 5 minutes learn to swim and then win a gold medal at the next olympic games. Prove me wrong and make your point.
Oh wait, you need years of training, natural ability and incredible physical fitness to do that? You mean you can't just go out and in 5 minutes be a completely competitive olympic swimmer? Despite everyone on our very "mortal" plane being human? Not to mention that even when you do, there are people who are exceptionally good at it who will just be better. Regardless of training for a long time, you're never going to catch up to what they could do. This is also a vice-versa scenario: If you are the best sometimes others cannot catch up to you. Regardless of how long they train or similar.
If what you were saying is true, we'd have a race with eight people and all of them end the race with a photo finish. How often can you say this ever happens in sport? How often do the All-Blacks (NZ Rugby Team) lose to Scotland? There are clear peaks in ability and it is clear despite everyone being human, that some people are just more able than others. I refuse to believe your argument that in a fantasy world where we have a clear marker of exceptional ability:
Being a player character in the first place shows that not everyone is equally able to learn anything anyone else can do.
If you read your PHB again, you'll see that
Player Characters are exceptional. There are few like them. In the same manner, some NPCs can have abilities that PCs cannot have because they too are exceptions. 4E is literally an exception based system when it comes to powers. Sometimes NPCs are going to do things you cannot do: But there are lots of things PCs can do that NPCs are
never going to be able to do either.
The major difference between a dirt farmer and a 30th level Wizard isn't fate or birth order or blood or genetics or radiation exposure or wealth or gender or hair color, but choices and training.
What's the difference between a peasant and a sorcerer - by fluff? I'll tell you one core thing: One person was
born a certain way.
I guess that's a particularly modernist, American, idealistic take on the nature of (most) heroes in D&D
It's not even true in real life and it's definitely not true in fantasy. Many heroes are born with a specific destiny, or more physically/mentally gifted than others or just
born with magical power like a sorcerer is.
So if the only difference between a 30th level Fighter and a stableboy is how many goblins you've killed, the difference between a 29th level Fighter PC and a 29th level NPC fighter should be even smaller, and, presumably, their abilities and statistics should be close-to similar (though I doubt anyone will do a detailed accounting of the exact numbers, they WILL notice big deviations from the norm).
A level 29 fighter is absolutely nowhere in the same ballpark of power as a 29th level PC. A 29th level PC fighter would
demolish the level 29 fighter. It wouldn't be a contest, it would be a one sided demolition. If the 29th level fighter was a solo, he'd be closer to the PC in power than an ordinary level 29 fighter - but still wouldn't be (equivalently power wise) as powerful as the PC.
PCs are absolutely exceptional, but NPCs can do many things they can't for numerous excellent reasons (too many to actually list). One core difference in our possible approaches is that my NPCs have frequently learned to do things because:
1) They were just born that way (Magical marks or similar, maybe even
made that way by someone else). Often like a sorcerer or for whatever other reason.
2) They've had years of study in what they do - learning their specific art is not something you can pick up within an adventure. An NPC who has spent 20 years of their life on something - to be thwarted by the PCs of course - is not likely to have relevant powers to a PC. A PC does not have the luxury of sitting down for 20 years and learning how to do what the NPC did.
3) Something else entirely beyond the NPCs ordinary capabilities lets them do that (such as a magical object given to them by a daemonic patron, which also horrifically sacrifices said NPC brutally when they are defeated to ensure the secret does not get out).
And many many many other reasons. Basically the PC is trying to ask me "Why can't I be an olympic swimmer in five minutes". The answer is pretty obvious why that is the case.
At the same time given nobody has ever asked me this and everyone is quite happy with the concept that
NPCs are not PCs in this edition, it has never come up. The only things I've had to explain were enemies ritual magic, which I pointed out took them roughly 120,000
years to finally put into place*. I then noted to the PCs they just ruined a ritual they had one shot at after 120,000 years and they were immensely satisfied with that (that's a pretty significant accomplishment). The only other issue I had was with healing NPCs: They aren't heroes. They can't take anywhere near the abuse of heroes. Having to explain why X guy is going to die despite your magical attentions can be quite hard. Ironically this is the opposite to any of your points, this is more "Why doesn't this guy function like we do?".
The answer to that is simple:
You're heroes and ordinary people just aren't like you.
*Immortals are really patient - they live forever after all.