at the time, Earth, cut off from the rest of the galaxy by warp-storms, robotic rebellions and other catastrophes had fallen from an ecumenopolis into post-atomic savagery.
it had been nuked, bio-weaponed, had it's oceans stolen, and was just generally a bad place to be, but it was still chock full of tech to loot, so the survivors crawling over the wreckage in places like Urush and Merica had plenty of laser guns to shoot at each other.
Techno-barbarians were chic and in fashion as mutants, psykers and such dominated the bulk of humanity and forced them to fight for them in ever-widening battles.
it takes like 30 years of effort by master geneticists to make a single Custodes, and while the golden boys are peerless, they're only ever available in numbers needed to guard the Emperor himself.
so the Emperor looks around at how shitty the world is, says "Right, I'm in charge now, you can't be trusted to run things" and immediately runs into the problem of everyone else is creating these mutant gene-warrior soldiers to fight for them.
they were also unstable, in the sense that their genetics broke down rapidly and they started to go crazy as their brain chemistry altered unpredictably or get huge cancers.
using samples of the Primarchs, the emperor and his scientists created the more stable Astardes process, which did take longer and produced actually smaller and weaker warriors than the Thunder Warriors.
indeed, they even included some handy brainwashing to prevent all that troublesome instability that had so plagued the Thunder Warriors with their mostly human minds suddenly being altered unevenly.
how did the Emperor deal with his surplus warriors? Did he: A: Throw them into battle after battle, letting attrition deal with his problem. B: Engineer the thunder warriors back into mortal soldiers. C: Stabilize them by turning them into Space Marines. D: Have them all murdered by the Custodes after a major battle?
Rolling out the first Astardes legion was immediately follower by throwing the Thunder Warriors into a series of grinding, terrible battles, capped off with their mass execution
lots of the Space Marines and Primarchs got very quietly but intensely worried that they were about to be Thunder Warriored as surplus to requirements and replaced by something new.
anyways, there's a guy named Cawl who's actually a collection of brains in jars working together who after 10,000 years finally worked all the bugs out of an Improved Space Marine line.
Finally getting them back up to Thunder Warrior size and strength, enhanced durability to make up for being bigger targets, and only a slight increase in the odds of a total systemic meltdown, and that's only if you've got them entombed in a Dreadnaught.
He had about half the primarch project info and a bunch of stuff from the Luna gene cults who were the people eho helped (ie were violently compelled) to help big E mass produce astartes
A: Throw them into battle after battle, letting attrition deal with his problem.
B: Engineer the thunder warriors back into mortal soldiers.
C: Stabilize them by turning them into Space Marines.
D: Have them all murdered by the Custodes after a major battle?