They essentially killed off all of the strong men so only the weak (potential slaves) would remain. You can't have a bunch of thinking, hardened, proud, yearning-for-dominance people running around competing with you. They learned that lesson after WWI.
Had they not done this, had they just let those masses of German men who sat through the conditioning they received from all of the propaganda of the Third Reich, it would've happened again for the Jews and the west. Those men were turbo-charged with pride and love of their own, men who once again were actually put off by the Jews cheap, plastic offerings; men who were starting to stand on their own legs again and actually love that which is beautiful and worth trying for, rather than that which is cheap, easy, greedy, unstable meaningless emotions.
They killed off the real Germanics; the wheat of the Germanics was thrown in the toilet and the chaff was left, because there can only be one brand of wheat (master) and one chaff (slave) in any one land, and nations had become outdated in the international world; a world the Jews wanted to fully impose themselves in every nook and cranny. They banned Nietzsche after WWII, and a whole host of other great books that made that race what they were, to make sure no chaff morphed back into wheat; they needed time to sow the salt into those fields, just like Carthage, to make sure nothing ever grew there again.
They needed time to degenerate the remainder into brain-dead retards that would go gaga over a turd like David Hasselhoof in a leather jacket with LED lights flashing:
This was their half-way solution, better for them than the Morgenthau Plan. This one kept the country industrialized with Golems; slaves; go-bots. They just got rid of the ones who would be a problem, and the denazification filters worked perfectly for civilians as well.