NPCs dying easily and NPCs being threats are both part of the fundamental crappiness that is Apocalypse World -- an NPC that is both alive and not a threat is a complete reversal of that crappiness, a major shift in the nature of the world (on a local level.)
If one of the former happens easily or randomly or without much specific fictional support, that's not a problem, because they have the fictional support of the entire game/world/setting behind them -- if the second happens easily or randomly or without much fictional support, there is no secondary support at all, and in fact quite the opposite. Therefore, in my experience, it seems to be more of a problem.
My philosophy with Ally stems from my interpretations of the Ally move, and in playing the game with Allies cropping up. It's based on two pieces in book, one about playing to find out what happens, and the second about there being "no status quo".
When I play the game, I don't know exactly what the various NPCs are going to do, how they're going to react. Their threat type has their impulse and moves, which helps me keep orientated and gives me some inspiration, but I try to play them like actual people, with all kinds of human emotions.
These individuals are all hard from a harsh world, but at the same time, I keep in mind that they are subject to changes of mind and heart. I don't know if they'll change their mind, change their impulses. I play to find that out. It's why I don't connect with your "fictional support", people make judgments and decisions and changes all the time, sometimes taking years, sometimes in moments. These decisions are arbitrary and not necessarily predicated on any particular event. Sometimes they're a surprise as much to the person as to the recipient. People are messy, and they don't necessarily follow any kind of narrative momentum.
I also don't view the world as fundamentally anything. I view things in terms of status quo. Right now the Apocalypse World is a horrible place. The only way to truly ensure that the person next to you isn't going to victimize you in some way is to threaten them, bribe them, or shoot them.
But it is a place that can change. I figure that If I can make moves to bring home that apocalyptic badness, then the players can certainly use moves to make things better. This opportunity to shift the nature of the people in this world, and to shift the nature of the world, is the ultimate expression of "No Status Quo".
Now, in my opinion, it also adds an element of hope, which I think is quite useful as a contrast to the starkness of the setting. Without a sense of hope that you MIGHT be able to change the people in this world, then I think the game becomes a lot less interesting.