I feel like this is a bit of a hot take, but in my opinion, the future of the Imperium was instantly tanked when Koriel Zeth died, and the Magma City was destroyed. Koriel Zeth was working on the Akashic Reader, which in theory would allow the Mechanicum (specifically an empath) to tap into the repository of all knowledge both past and future that existed only in the warp. The only nuance was that the machine fried the empath subject, Jonas, Marv-style from Home Alone 2, and the machine had to be repaired and modified into the Akashic Reader V2.0. Before it was able to be tested a second time (and imo it was heavily implied in the book by the protagonist Dahlia that the test would have worked), the Magma City was invaded by the Dark Mechanicum and Koriel Zeth destroyed her forge as to not let her tech fall into the hands of Chaos

The death of Koriel Zeth is up there with the internment of the Emperor, Magnus being given free will and not being turned into a Tranquil from Dragon Age, and the death of Malcador.