the NPC will be coming. FAR too much time has been spent by Lemmy getting them right, acting and behaving the way TIS want and in a manner consistent with George Romero Lore, which is the games Lore.
unsure why you think raiders and insane people would be outside the scope of possibility. too me, that sounds quite possible.
fact is, their are people who would rather use their guns and physical advantages to just take what others have saved, rather than actually loot themselves. and i mean in Real Life. throw in a Zombie Apocalypse and people would change, in some it may be literally overnight. in others, they may have just been waiting for a reason to go nuts and start stabbing, shooting and killing everything in sight. then you have the ones who are so scared shitless, they may accidentally kill you if you approach from the wrong side while they have a gun and they just squeeze the trigger without checking if you are "alive" or Zombie.
as for insanity....try to truly imagine the horrors you would witness each day. even the strongest mind just needs a little push, something that mind just cannot fathom, and over the edge we go into Insanity.
and lets not forget the "real" possibility that a cooperative group of players and NPC has their base swarmed when a damn helicopter leads a horde to them. now these people just lost everything, and will need to restock/resupply with whatever they can find. this kind of thing could easily lead to a formerly friendly group turning to raiding tactics, if only to survive one more day, eat one more meal.
NPC have been a target for a long time. no way TIS is just going to say screw it, and throw away all the code that has been written for them.
instead of getting worked up over what MAY happen, just keep waiting with the rest of us and we will all find out together how it all plays out.