I suppose so. But from my perspective, the more likely outcome is that they'll have Rowboat Girlyman turn progressive, since he already hates the Imperium as it is:
"Why do I still live? What more do you want from me? I gave everything I had to you, to them. Look what they've made of our dream. This bloated, rotting carcass of an empire is driven not by reason and hope but by fear, hate and ignorance. Better that we had all burned in the fires of Horus' ambition than live to see this.
We failed, father. You failed your sons, and we in turn failed you. And now, to compound our arrogance and vainglory we have failed all of them too. Did Horus not say you sought godhood? He built his rebellion on that claim. Oh how he would gloat and laugh at the state of the imperium now."
He's already GW's answer to Rey; a hero who has everything in his power to change things for the better. So once the traditionalists and the back-stabbing ass-kissers in the Imperium turn against him and launch a civil war with him, Guilliman will use it as an excuse to purge the Imperium of every backward, retrograde, traditionalist influence, and GW will use it to purge any "problematic" elements in the Imperium and have Guilliman be Mr. Rational, arranging an alliance with aliens like the Eldar, while having Belisarius Cawl innovate more new tech for the military. Guilliman will then go off eradicating the more fundamentalist parts of the Imperium, to create what amounts to an enlightened despot state that the normies can love without being called fascists. Instead of Space Nazis, they'll have Frederick the Great in space, now with more tolerance for those filthy xenos.
Mark my words. They'll turn 40K into an SJW agitprop eventually. Especially since it makes them lots of money, ever since 8th Edition dropped:
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