Hi all,
First off, forgive Wak for being casual. I go by "Russell" when I'm writing, but "Rusty" in day-to-day stuff. I'm Russell/Rusty Zimmerman, not Rusty Childers. I promise.
Second, as has already been pointed out,
Land of Promise -- the only one in the Tir Tairngire series of demographics that
I worked on -- actually shows a population
increase, not decrease.
Third, I talked to some poli-sci and history folks about it, yeah. But, to clarify, that's because my office was right down the hall from theirs, and we ate lunch together about once a week anyways.

I'm not sure how it turned into me doing extensive research and multiple interviews, but I want to nip that in the bud. All I did was talk to some buddies on campus. I wanted to get a few ideas about how quickly I could get populations back
up, because I, frankly, agree with you that their "low point" was a bit too low. I wanted to restore stability (in-game) and playability (out-of-game) to the setting, and getting the population to level off seemed like a handy way to do that; to show a dramatic increase from that low point, to still not be a nation fully recovered (and back to original
TT levels) but rather one still scarred by years of harsh internal policing, but to clearly be a nation on the rise. So, yeah, I explained a few things to a few buddies from elsewhere within the department, had them crib me up some ideas about how quickly populations might recover from that sort of thing, averaged out their two answers, and went with what ended up being almost a nice, round, five million.
Fourth? As for what was "demonstratively skipped," and what went into the book? Frankly, you're the first person in all the time since release -- which is what, three or four years, now? I'm a little brain-fried from the GenCon drive, sorry -- that's even mentioned the demographics, much less that's said "Man, I sure do wish someone had given me more back-tracking story info, and less about the
now to work with, play with, and run a game with."
My job, and my goal, was to make the Tir playable again. I wanted to let folks get thrown back into it, and to introduce them to a new generation of Tir Princes (except ones that aren't immortal and thousands of years old, this time, but, rather, ones that actual players might actually interact with and work for and maybe even challenge without it being an auto-death, which was the "some kind of shadowtalk about 13 persons").
Land of Promise didn't have the budget or the roominess of a 165 or 175 page sourcebook. It was initially supposed to be the supplementary text to a series of adventures (basically just an expanded GM packet to the
Elven Blood Missions arc), but I lobbied to extend it into a purchasable e-book format, and basically just added to it and added to it until The Powers That Be made it available as an update to the prior books. An update, not a replacement.
I'm sorry if you don't like the final result, but I can assure you, the last thing I did was "remove" anything.
Lastly, I think that if you take too hard a look at any of the demographics info -- in basically any
Shadowrun product, from any edition -- you'll find you're running into some problems. We used to call this sort of thing "FASAnomics" and laugh about how silly the
Shadowrun and
Battletech universes were, and the plain reality is that it's all still pretty silly. At the end of the day, you write up numbers that make sense for the story you're trying to tell and the setting you're trying to describe, and that's that.