Many people in this sub throw around certain excerpts such as these from the Dark Imperium trilogy:
>'Into the warp I went, fifteen thousand years ago. Cast adrift by the storms that wracked the galaxy as man’s apotheosis drew near. Deep, deep into time I was sent. I have seen the beginning, when the warp was first breached and the slow death of the galaxy began. I have seen the end when Chaos swallows all. I know the fate of mankind. You are not equipped to prevent it,
and
>‘I am Qaramar of the Lost Second,’ said a rasping, hideous voice that came from nowhere and everywhere. ‘Last Watcher of the Last Moment. Fifth in Nurgle’s favour. I cannot be killed. I have seen the end of time. I will be there when the final atomic motion of this hateful realm decays into blessed entropy, and Chaos will be born anew. I am sent here to be your executioner, anathema’s get.’
However, the future is not set in stone. Sanguinius' vision in the Dante book states
>There shall come to pass days of great darkness, when mankind is diminished and all the lights of the world shall be extinguished, and the final scraps of hope torn away. I dreamed I was upon a plain of black sand studded with diamond stars. In the dream there was a great hunger that pervaded all time and space, a more terrible and consuming appetite than the thirst that dogs my sons. It rose from the east of the night, and swallowed the moons of Baal that coursed across the unfamiliar sky. Before Baal Secundus was consumed, a bright light flashed upon it and sped away, outpacing the shadows.
>The hunger spread rapidly, bloated by its meal of my home. Fortified by the blood of Baal, the formless hunger took shape, becoming a ravenous dragon that consumed the stars in great mouthfuls, until the only light was the memory of their glory, trapped in the diamonds on the sand. As the last star was eaten, the hellish Octed of the traitors burned through the western sky, writ in fire on the starless void. Then this too went out, and I was alone in the dark.
>Shadows swirled and parted. The vision lost its disguise of metaphor, and I looked upon a scene that may be a true echo of the future. I saw my father. Ruined. Broken. I knew it was Him, though His body was little more than a corpse, for I could feel His mind. His power was much reduced in potency, and I could feel no sense of consciousness there, merely raging, ungoverned power that threatened to obliterate my sleeping mind. This living corpse of my father was trapped in machinery that fed His soul essence of others. I do not know if I should commit this to paper, even in my private writings. He cannot ever know of this fate, if He does not already. Or is He aware, and makes this choice between that life in death and the utter destruction of mankind? If so, my respect for my father grows...
>
...
>The hunger came for my father. The puppets of the Dark Gods clashed with the hunger for the pleasure of killing Him. There was a warrior in gold before the throne, surrounded by my father’s Custodians and other heroes who, mighty though they were, paled next to the lords of our days. There they fought, and there they died. The vision ended as the devourer of flesh and the devourers of souls closed in on my lord and creator. There was despair only, despair and more despair. But before I woke something more. I sensed stirring in the warp, and the touch of my father, His mind made anew, and the knowledge that all might be well.
>As I am fated to, so too did this golden warrior lay down his life to protect my father. The precious seconds he bought with his blood could change everything, or they could change nothing. Maybe the vision is false. I pray the future is mutable, and so it has proved in the past. All but the moment that draws near, the reckoning when I must face my brother. That I cannot avoid.
Guilliman's resurrection was never fated to happen, yet it did, and Baal still stands despite Sanguinius' visions. Chaos can say that their victory over the galaxy is inevitable all they wish, yet until it happens, it isn't true. While the warp is a timeless realm, the Materium is not, and the laws of causality still hold true in realspace.
Out-of-universe, while BL authors can talk endlessly on how they foresee Chaos or any other faction winning, those authors are not GW, and their preferences/headcanons/whatever you want to call these musings are not any more valid than yours unless GW backs these statements themselves.