There is no humanity on the other side of the fence. Abaddon isn't human and his victories, and those of Chaos and in general, are just as much defeats for humanity as the military losses the Imperium suffers, if not more so.
Well I regard all chaos worshiping and following humans/astartes as humanity or at least its legacy, being on the other side of the fence, a future for mankind under the yoke of chaos (I am not sure but this is prob what abaddon wants for mankind) is a fair possibility for mankind, I liked the idea that the other side of the fence is equally fckd. It is not because Abaddon is non-human anymore that all chaos worshippers have no human affiliation anymore, a lot of NL for example are very close to the original astartes template,
Then look at the Gaunts Ghosts or Macharius novels. Cain also regularly triumphs over the forces of evil. Further most Space Marine novels end with them winning the battle, even if Abaddon still manages to advance his ultimate goal.
I have not read those books to be honest, and the IOM indeed triumphs often wins but often a pireas victory where again and again they lose ground. Give me an example of a IOM victory with the same gravity as what honsou did with the gene-seed storage, Talos did with the Astropaths, Honsou at ultramer, Abaddon with the GK, the only I can think of now are the destruction of the dies irea (or somethin like that), or Alaric, the former I do not regard nearly as significant, the later not so sure how to position Ben his GK nowadays,
The problem your having is that M999.41 is the eve of the 13th Black Crusade and is therefore the Imperium's darkest hour, they can't be winning in their darkest hour.
, Again, I do not mind the IOM is going down, it is like seeing a boxing match, I just want to see a cool fight, not what is suggested in the recent lore, I want it to be undecided until the 10 th round.