Orion,
Glad that you found my post of help. There's something about your concern I'm having trouble wrapping my brain around, as in I'm not quite seeing what the problem is, but I'll have a go.
First off, it still sounds like you are most concerned about the MC as the source of fresh material and going's on. This doesn't really sound like Apocalypse world to me, at least not the MC as the major or even primary contributor. The 1st session is not only about defining the world, but discovering what about the world is interesting/engaging for the players. They lead the game, not the MC. The MC just takes what happens in that session, and then says "ah, these non-PC things happened because of this" and makes that be something that's the players have shown is interesting. So while there is prep work done, it's much more about solidifying things that are alluded to during that first part of play.
After that, I don't really see a lot of ongoing prep. The MC spends most of his time turning the existing screws, not adding new ones. If you'll note the examples I gave, they were all accumulation upon existing fiction or changes to relationships with them, not the invention of completely new things. So yes, there is a certain burden of providing new information, but experience with these sorts of games shows that there is almost always a completely natural and exciting path the game takes from where it is before, especially if the players are fully protagonized and pursuing story that interests. Change, after all, doesn't necessarily mean destruction.
Your concern about the PC's wiping out the entire cast of named NPC's also doesn't ring true to me. While theoretically possible, I would guess that this rarely if ever would happen in a game like this, it certainly wasn't happening with my group. The idea of PC-NPC-PC triangles, for instance, means that while one PC might want to kill off a character he's more than likely fucking with the another player's character by doing so, which will have it's own repercussions. Also, terming NPC's "threats" does not mean that they are inherently people to dispose of. Pariah's daughters are a threat to his life in that one of them spends her time trying to help him in his goals (and fucking up pretty badly) and the other has started being able to hear peoples thoughts and insists on telling everybody about it. Is Pariah likely to just put a bullet in both of them and call it a day? not likely, but they definitely are threats to him getting what he wants/needs.
So I don't think coming up with new material is as much of a concern as it might appear. You'll have a number of things to keep the PC's occupied, and like has been pointed out before, the likelihood of the PC's clearing the decks in a single session are slim to nonexistent, especially if the MC is working those triangles so that they aren't always exactly on board with each other.
You also seem to allude that improvement makes the turnover of story elements faster. I don't buy that. Limerick seduces Kipper, getting her to sleep with him (fun in itself) but more importantly giving him a set of ears on the inside. Great: he succeeded because he's gotten good at seducing people. But now he's got to prove he's not just using her, and she wants out, so he's got to figure out how to string her along until he finds out what he needs, and then maybe, MAYBE he'll pull her out... As has been said before, failure is interesting in this game, but success really should be JUST AS interesting. Being able to do what you set out to do more often doesn't mean your life is any easier.
Or if the player takes new moves: It means they have more ways to try to get what they want, but it doesn't a) guarantee success or b) mean that success is without consequences.
But I don't think there is any logical argument that can prove this point to you. As I said before, what you've pointed out is logically possible, but I would argue in fact highly highly unlikely if not downright not going to happen. Maybe someone does in fact min-max to the point where they are relying on only a couple stats, and those stats are being rolled at a ridiculous rate. Ok: what that buys them are a few more points, and some new abilities, and maybe new outcomes that lead the story in new ways, but they don't give them a pass on dealing with the "reality" of the game. You take an action, you succeed, and that means things happen, not all of them good for you.
finally: the rate at which players "PCs kill named characters, solve threats, or acquire improvements" does NOT go to infinity: it has a very real limit to how fast that will increase, even just looking at the rules and not playing. Even if you manage to collapse all rolls to just rolling one stat (impossible), unless you find a way to constantly increase the number of rolls you make (again, will hit a hard limit in play) AND decrease the number of rolls between taking improvements (not possible with the base ruleset, though that can be hacked) there is an upper limit to how fast a player can advance. As they advance, the list of options they have is limited: The lists in the individual playbooks are limited, saying "mark this once" then the ungiven future choices are also limited. Eventually, that character will run out of ways to improve.
As for your call for empirical data, well, all that can be pointed to are actual play sessions. I don't think anyone has collated a statistical analysis for the relative speeds of advancement or a measure of the fun had. As has been said, experience playing is the only real way to get a feel for what works and what doesn't. But again: the game has been pretty thoroughly playtested, and of those that have played no-one has seen this as an actual concern, rather than a possible one. Perhaps your experience will be different?