No, no I do not expect to get a full picture of society from any given npc and have no argument that any picture formed at this state of the game is definitive but to say that the quests are self-contained and unrelated to each other is to say they are pointless and only there for the reward. I know that is not exactly what you said UTM bit it follows from your last two sentences. Fortunately I do not believe that that is true, nor do I really think that is what you meant.
I am not positive how you go about quest development but I think you probably take an idea for a particular npc and then story board it, pass it around to your colleagues for input and then start to implement it. At least I would hope that was close to the process. If this were the case then those people who have written content for relevant npcs would be able to help shape it, if they are still around. In this way there will be some internal consistency to the characters and the whole will be more than its parts.
I was not suggesting that we need fully reactive npcs, just that as much as possible they must conform to the same laws the players do. I really do not expect you to write any of them out of the future. They may move to new sections of the world as those sections get developed and even get disguises or lose old quests or quest connections. That would be completely understandable.
I hope you can take this thread and others like it as complimentary because it shows your work is engaging enough to think about and not something to just do the spoiler to get the items and never give it another thought. You do not have to agree with the thoughts expressed but it cannot hurt to consider them as an unexpected reflection.