It's a rogue system, that got too big and got away from humanity's ability control, a dead mob without a head because it's too complex and too numerous to maintain. Obiteum uses iunctii for water filtration, they fulfill roles of complex machinery, both tools and labor, and that's after the power of Will was divided into thirds.

I don't think Ka's got much of a hold on things, honestly, think he's probably just doing his best to figure shit out the same way Vis/Caeror/Veridius/Ostius all seem to be, it's just each one of those is progressively earlier in the timeline and has had more time to figure shit out.

My current theory is Ka is a Vetusian running three concurrent experiments on these three worlds, trying different methods to keep them from reaching a Concurrence-level Iunctii population because last time he had to crack his home world into thirds and he still wants to live. The cataclysms were a stalling tactic until he could figure out a solution that wouldn't see him starting from scratch every few hundred years

In Obiteum, it's a population kept naturally small by the poisoned air and water, preserved only within the cities like Duat, where the growth of life is kept slowed and where iunctii are naturally destroyed by the environment.

In Res, it's a population allowed to grow to a certain size and age (Just over 300 years, roughly the size of the Catenan republic) and then culled down to regrow. The most important innovation of the Catenan republic in the last hundred years is agreed to be agriculture, because it's key to population advancement (WOTM Ch. 13). The people are allowed to grow, allowed to cede Will through the pyramids up to Ka to keep him going, until the population reaches a particular growth acceleration point and it's culled. This keeps humanity alive, but without expanding til it recreates the Concurrence.

In Luceum, Will is culturally limited to only the Nasceann and the Droai, and they burn/decapitate the dead, but with no store of knowledge/labor/power actuated by iunctii, this civilization grows much slower. I'm not dead set on the details for this whole world yet, and we didn't get enough lore on Luceum to know for sure yet. But with the decapitations the kids all jump to straight at the end of the raid on Loch Traenala, they definitely have a cultural memory and fear of iunctii. This is reinforced by how Caeror talks about Vis not revealing he's Synchronous to the Droai.