It wouldn't work?
All indications are that the geneseed is at least partially an inherently warp active thing. You can't make a Null Primarch and trying to implant regular geneseed into a Null would just result in rejection.
It also pre-supposes that the Emperor didn't plan the Heresy. Because everything he did appears almost purpose made to put him on the Golden Throne and worshipped as a god by the only species still able to manipulate the warp on a galactic scale.
The Space Marines, and Primarchs, were designed to conquer the galaxy and wipe out potential competing powers and ideologies. None of them were made to rule or create a stable state.
The Imperial bureaucracy was an absolute joke that barely even pretended it was trying to be anything but an autocratic dictatorship with a schizophrenic bureaucracy. This despite the fact that the Emperor has a super human intellect and has lived through basically all of human history; if he wanted to create a stable polity he absolutely could have done so, and yet he was manifestly uninterested.
The Imperial Truth was a blatant farce, and for a man who professed to despise religion EVERYTHING he did seems purpose designed to ape religious faith. You don't make yourself a four meter tall adonis clad in golden armor with a halo around your head and a psychic field that makes everyone you meet see you as the most absolutely perfect being and emperor to ever be conceived of if you are trying to deny your divinity.
You don't let Angry-Lad, BATMAN!, or Emo Cyclops behave the way they did and get away with their antics if you actually care. Angron should have been tossed in stasis or killed, Curze was a complete whackjob, and Magnus should have had his entire legion put to death when the flesh change showed up before getting himself shackled to the Emperor's side for a good education in why warp fuckery is not to be fucked with. But, again, the Emperor never gave any indication that he intended long term viability for basically any of the Primarchs or legions.
Ten thousand years with quadrillions of beings praying daily to the God Emperor, ten thousand years of every human soul being dedicated to the Emperor upon their death (the number who don't sincerely and fanatically worship the Emperor is literally so small as to not even be rounding error on the scale of humanity), a thousand psykers per day fed directly to the Emperor. Over ten thousand years that is 3.65 billion psykers directly sacrificed to the Emperor.
By everything that we know about the Warp, the creation of Warp entities, and how belief shapes reality in 40k; the Emperor is being fed a feast of psyker power beyond anything that any entity has enjoyed since at least the height of the Aeldari Empire. You think that occurred by accident?
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If the Emperor wanted general purpose Null soldiers then he would already have them. He created the Sisters, he created the Ordo Sinestro, he probably created the Culuxes. All three were highly limited forces kept under the Imperial Households direct control and only deployed in very limited manner for specific objectives; virtually all of which were internal policing operations.
We saw what the Emperor did when he decided a specialized anti-demon force was needed; and it wasn't blanks. It was the Grey Knights.
