I suppose I understand that as a player, but it simply doesn't seem realistic in a role playing game. If the world is dangerous, then the would should be dangerous. If your questgiver dies, sorry for your luck. I don't think that the entire wor4ld should revolve around the player character. I like the idea that the world goes on without my direct control.
I like having the world existing outside of my direct control, as well. I just think there should be limits. Someone dying way on the other side of the world map because I wasn't there to do anything about it - I don't see that as adding anything to the game. It's punishing me for not babysitting all the important NPCs. I'm not against time-based stuff going on, (didn't bug me in the old Quest for Glory games, for example.) But a quest-giver getting killed by a random spawn without my knowledge, or even giving me a head's-up to let me know that something's going on; I don't see as adding anything to the game. I'm of the philosophy that gameplay should come before realism - I think that falls into that category. (Though at the same time, admitting that's a subjective view - just my own preferences...)
I'd probably be okay if there was at least a message telling me something was going on outside of my current location - or better yet if I was given a chance to decide whether I wanted to intervene or not. As an example - Mad Max: Road Warrior. Max comes across the oil refinery refugees besieged by raiders. Were that a videogame, the PC would come across the situation there. The stage has been set. If he goes off and leaves the place alone, I wouldn't have a problem with coming back later to find that the raiders have wiped out the settlers and taken the refinery over - slaughtering all of the potential quest-givers in the area. Because I could reasonably assume that events would progress with or without my intervention.
What I would have a problem with would be not coming across that encounter until 3/4 of the way through the game; only to find out that everyone is already dead. To have completely missed any opportunity to do anything meaningful there through no fault of my own; or having never even known that anything interesting was going on there. To have not even recieved so much as a distress call alerting me that there might be something interesting there worth checking out. It would make for a better simulation - but (I don't think,) not a very good game element. Not to mention it would have made for a very short movie.
Sure, the game developers could work in some contingency plans for picking up a plot after the fact - finding a survivor and getting revenge or something. But that's more work for the developers - and by now we've all agreed, I think, that they're working with limited resources.
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A good story need not rely on npcs to live. Quests don't either.
Concievably, you could have a story or a quest without NPCs - but without people, that's kind of hard to do I think. I guess you can always pick up a story from a holodisk or terminal, but I think it's (generally,) more compelling when you're reacting to human beings. Going out and completing a task because a computer led you to it works, but I generally think it's better if you're reacting to an NPC that you have some sort of emotional connection to.