I feel like the loss of a primarch was one of the larger events of the Heresy leading up to the siege itself. Maybe not the largest, but certainly meaningful and memorable. With the timeline having progressed in the way that it has, it seems the death itself received little attention (not a criticism, it was the Phoenician's story and not Ferrus') and the impact of the death seems to be minimized (the Hands continued with business as usual).
I don't know that a full novel would be appropriate for this, but it seems that there should be something - even a 50 or 60 page short story - to deal with a legion losing their gene-father, leader, inspiration, and battle leader. The veterans of this legion cut off and replaced their body parts in tribute to and emulation of Ferrus Manus. I'm not saying that they can't regroup and come out fighting again, but I feel like there is a story to be told. Of a legion coming to grips with their father's death, re-forging and ultimately moving on. I'm guessing that the upcoming Salamander novel will have some of these themes in their search for Vulkan (I can virtually guarantee that the words forge, anvil, and into the fire will be used). So maybe it was decided that two separate stories like this would be overkill - and that's fine. But again, only one primarch was lost before the siege, and we don't get any feel for the depth and/or impact of his loss on the legion.