It is a balancing act.

That specific faction doesn't want the peaceful/high-IQ gone, they just want them cowed, subjugated, and effectively a slave caste and managerial caste all in one, kept meek and afraid by violent and savage monsters all around, which that faction can sell "protection" from, while simultaneously funding more and more of the savagery.

They don't want to rule a wasteland, they want to rule a medieval barony, literal robber barons, "protecting" their few valuable serfs from the barbarian hordes. And to make that world a reality, they work towards making the barbarian hordes, and making the serf caste.

Too many barbarians, and they die too. They don't want that. They're evil monsters, but they're not so stupid as to directly desire their own destruction. But too few barbarians, and those serfs might become merchant princes, might create a rival power structure, might grow and threaten the Eternal Rulership.

You can see this pretty cleanly in areas with accelerated or further-down-the-line "progress" in wokeanda-ism, like many south African countries, where they want killing white people for sport to be legal, but ALSO want white people to run their farms and businesses because the moment the white people are all killed off for fun, the entire country collapses until they beg and plead for white people to come back and promise for realsies this time they won't kill them again. Which happened twice down there.