From what I heard, the last of the Primarchs died 1500 years after the Horus Heresy. Which I always thought to be Rogal Dorn.
For the question of Dorn not reforming the Imperium, he had already once been to the brink for the refusal of adopting the Codex Astartes, but relented before a new scism was started. The fists was even fired upon. And it weighed heavilly on him that he had failed to protect his father and thusly threw himself into task of hunting traitors, and why the Imperial Fists legion willingly went into the Iron Cage, both to absolve his guilt by facing declared traitors, and to whittle the legion down so it could be broken into chapters with much more ease. And the Imperial Fists have a long known history of masochistic behavior, which I belive stems from Dorn's guilt over failing in his duty. So it is little suprise that the demigod went to his death in a battle barge filled of ravening culltists, he had little more to live for. His father was between life and death, the imperium was degenerating.
Look at the Excoriators and the darkness afflicting them, said to be a physical echo of the despair Dorn was feeling over the loss of the Emperor. In Prince of Crows, Konrad Curze is having what appears to be a vision of the demise of Dorn, pulled down by cultists as knives draws his divine blood.