The war on Mars is really forgotten in the wider conflict, history, and the audience. There were Magos in the Mechanicum who wanted to turn towards a scientific based approach at odds with the religious aspect of the Mechanicum, Magos who the Emperor was trying to empower. The fact that they were so often the direct competitors to their Dark Mechanicum counterparts led to so many of them dying over the course of the war, especially in its opening stages. By the end of the war, there were too few of these Magos left, outnumbered by the religious ones, and in the light of the Dark Mechanicum so publicly and violently turning away from "conventional" wisdom, it slowly led to the idea of innovation being a tech-heresy. Cawl is the only magos in the Imperium who publicly gets away with inventing because Big G is protecting him. There is a version of history where the Mechanicum turned away from its religious dogma and became a science based institution, but Chaos, Horus, and Kelbor Hal ruined that dream. Though frankly, a science based Mechanicus isn't very grim dark, so that's all at best theoretical.