Also, I don't buy the Cabal stuff. I feel like A/O have been totally played by the Cabal. Pretty much every other portrayal of divination in the Warhammer 30k/40k background shows many, many threads of the future, with hundreds of possible outcomes, yet here there were only two? I personally reckon that the Cabal wants to destroy humankind, and basically duped the Alpha Legion and showed them only what suited them for their goals. This is especially true when looking in hindsight and knowing that the Alpha Legion taking the path they did actually led to the outcome they were supposedly trying to avoid- what if the Alpha Legion had sided with the Emperor, and Horus had been defeated thoroughly? The Cabal intended that outcome IMO.
Ties in with how Chaos works.
Yes, the cabal wants to eradicate the human race. Because they want to destroy Chaos. Chaos and humans are tied closely. Chaos was not a big problem before the rise of mankind. They did exist and they had entire races to do their bidding but that was rather a minor annoyance in the face of Eldar/Necrontyr/Old One/Enslaver power.
Basically the emperor was on the same road and also could have succeeded in theory with his mission of removing all belief systems from mankind. But as the priest of the last church on earth told him "People will always believe".

Yes there are more fututes than two. But as the Eldar see it, they eliminate the most unlikely. And in this case the draw between the Emperor and Horus was highly unlikely. This was actually the only scenario in which Chaos did actually win and could exist further. In both other cases the Choas gods would have starved slowly and eventually get back to their former state of beeing powerful but irrelevant.