Chris brings up a good point, I want to add my voice to his. Essential quote:
SAY THIS FIRST AND OFTEN
To the players: your job is to play your characters as though they were real people, in whatever circumstances they find themselves - cool, competent, dangerous people, but real.
Sure, everything's a threat, but that shouldn't really matter. Lets see if I can get at this:
If a person has a kid, that kid is a threat. The kid could fall down a well. The kid could catch the fever. The kid could start stealing things. However, it doesn't matter. IT'S YOUR KID. You take care of the kid and love the kid and do everything you can for them. If you don't what's the point of living?
Kids are an easy extreme example, but don't think for a second that it isn't true everywhere and always. If you've got a hard-holder, their citizens are threats, but caring for their citizens is still their job. You can't have a hard-hold without them. The same is true for chopper gangs, hocus cults, etc. Or if you're none of those, if you're just a human being and they're a human being. Maybe they do a kindness for you, and then you've got a relationship. They're a threat then, but that doesn't mean you get to cut them loose at the drop of a hat. If you don't care for others, what kind of person are you?
So a real person cares about others. They're not just threats.
Simple example:
My skinner rescues a helpless woman about to be raped. Next session that woman shows up to the skinner's pad with problems. These problems become the skinner's, because she's invested. The would-be-victim is now a threat, because she makes problems for the skinner.
Then you take things to the next level and make triangles...
Example continues:
The hard-holder has a business associate. He's useful and reliable and the holder likes him. The thing is that he's the problem for this would-be-victim. Now the skinner and the hard-holder both have a stake in the situation, but they want different outcomes.
Boom! PC-NPC-PC triangle. Two PCs are involved in this businessman's life, but want different things. In my story, the businessman got murdered. It made all kinds of trouble for the hard-holder. Makes him think twice about the other PCs as his allies, now doesn't it? Maybe they're the real threats.
That's not an awesome example, but it's an easy one.
Back to your original question: is Apocalypse World limited in scope and quickly exhausted? (Perhaps like his other games.)
I've only played DitV and AW. Based on my experience, I think that your friend brings up a fair point about DitV. When I played DitV my reaction was strong, but I didn't find myself getting a lot of mileage out of it. I loved DitV, but I've only played it a few times. I didn't play it enough for it to get "tired", but I can easily imagine that happening. It doesn't take long to feel like you'd "done Dogs".
I completely disagree about Apocalypse World though. I don't feel like it will get tired easily. I feel like there are vast vistas to explore here. I think we'll finish my current campaign and then immediately start another one. We'll set it in a very different situation, with very different characters, and it'll be awesome. After two campaigns we'll probably take a break for some other genre, but we'll be back to AW. No doubt about it.
Now, I do think you're right that the NPCs are fundamental to making AW engaging. If those haven't been clicking for you, I strongly suggest you reread the MC book and put some effort into pushing the PC-NPC-PC triangles, threats as something other than enemies, and getting your players invested in more things than survival. I've had to work really hard at these things. It hasn't been my natural inclination, but all the tools are there at your disposal.
Of course the game isn't just the MC's creation. You also need to talk to your players. I know I've had to. I was sitting around planning a few weeks back when I realized that one of the characters didn't have anything in particular they cared about. They were like characters I've seen in most of my other games. No family, no job, no friends other than PCs, no real desires other than survival. I took the player aside and was like "what does your character care about? what do they want to accomplish? Powerful, dangerous, awesome people don't just hang around waiting for life to happen. Lets find something for you." It worked, they got it, we brainstormed, and now the character is much more engaged and interesting. You can do the same.