I hadn't explicitly spelled it out until now, but to my mind it makes sense. The Fimir are obviously an exceedingly top-heavy sort of race. The vast majority of their kind are Shearls, this is canon and known. Fimm Warriors are a separate caste, they are noticeably better equipped and larger. It's entirely possible for this to be partially a result of who gets what food and equipment, but it's also likely that they literally are just...birthed different, sort of like how ants have workers and fighters, or for a more Warhammer reference how stormvermin are born as blackfurs and clanrats..aren't. We also all know how valuable magic can be, so why would they not produce more Dirachs and Mearghs and whatever? Culturally, we know that in their current state in the timeline, a Meargh rules alone, any Meargh she births goes elsewhere to set up her own clan. Now, obviously this ties into inhuman mentalities, jealousy/envy/etc. worry over clashes for control and power. But given environmental concerns, pressures, etc. it is entirely likely that they don't over-produce them because they can't. Wouldn't you want to try and breed mostly Dirachs and Fimm, instead of just Shearls? And Mearghs? So there, naturally, has to be some sort of bottleneck. Partially resource based, partially because, I figure, there's a bit more that goes into it. If it was so easy for them to produce more Dirachs and Mearghs through the previously spoken very unfortunate alternative methods, then they should have way more than they have. Bounce-back should be far easier than it has been, i.e. possible in the first place.
But it adds up, they're pyramidal in levels of influence/power/strength. Mass of Shearls, atop which are the fewer but elite Fimm, atop which are the Dirach Cabal, atop which is the Meargh, the sole female and highest power over each individual Fimir Clan.
Anytime another Meargh is born, she is sent off elsewhere to have her own, separate Clan.
Now, to me, a 'golden age' (for them) Fimir Civilization would likely still be similar to that, just on a grander scale. Cities of dark stone and iron where the Mearghs rule as queens, the Dirachs as their councils, the Fimm as their elites, the Shearls as, well, everyone else. Possibly with subordinate Mearghs running districts of the cities, with their own Dirach Councils, within greater Ur-Clans, and individual Dhar-Mearghs as rulers. However, as their civilization and power degenerated, they collapsed further and further inward while maintaining a basic parallel to their previous civilizational structure.
It's noted time and again that Mearghs are, quite literally, the sole females of their race. There are no non-Meargh females. And it makes no sense, whatsoever, for them to be incapable of breeding up their previous true civilization beforehand if they were so direly dependent on humans. Because, again, the Fimir were BEFORE humans, it's when the humans showed up that things went to shit for the Fimir because they lost the favor of the Chaos Gods. So it quite simply is flat out impossible that they REQUIRE humans to build up fully, it's just that they CAN use humans.
But again, the problem with the Fimir building up their numbers majorly, rather than bitterly decaying and fading out, is that anytime they would try to do said build-up, they would get noticed. Fimir are pretty big, and require food and resources to maintain themselves, and any sufficiently large clan that would become a major threat would be treated as a major threat by the rest of the Old World, because after a certain point, people are going to start noticing them trying to range out for food. And I severely doubt them managing to perform some kind of wunder-agriculture in the swamps as they are. And unlike a hypothetical victorious Fimir on Albion scenario, they would never have a place of total defense to retreat to. Generally, Mearghs remain solely in their places of power, built up since they were first sent away by their mothers. On occasion, I'm sure that if a Meargh dies unexpectedly, her daughter can come back and reclaim the clan and add it to her own self-made one, thus creating a more powerful clan overall, and so on. But overall, Mearghs are hideously powerful, but are also vital to the functioning of their race. No Mearghs, and the Clan will begin dying out. Possibly the Dirachs could try a ritual to somehow manage to get a Meargh birthed through twisted methods, but efficacy is unknown, uncertain, but definitely unlikely.
So...yeah.
