Extra heresy: Maybe the reborn Emprah is so upset with the Imperium's perversion of his ideas, that he release Men of Metal 2.0 and rules them instead. Would solve the Chaos problem at least.
Even after so much time that's kinda out of question, because...
His original 'victory conditions' are all failures. Secularizing humanity? An utter failure. Using the webway to get rid of warp travel? A failure. Uniting humanity? Debatable given that the fiction is populated by a never ending series of rebellions and withdrawals from the Imperium as the basis for half its stories.
None of them are his actual victory conditions. Emperor's prime "victory condition", the entire reason behind his existence is ensuing protection of Humanity against outside threats - be it Enslavers and other assorted Warp monsters, Xenos, or such. As long as humanity continues to exist he has won.
IIRC in the Master of Mankind novel, he basically says "we've failed" after Magnus blasted open the webway gate in the Imperial Dungeons and ruined most of the technology the Emperor had built to make his dreams a reality, never to be rebuilt.
He has failed in short term basically, because Golden Throne was supposed to be one of primary prerequisites to ensuing that Chaos goes "go" and stays "go" (which is kinda necessary to safeguard humanity), and even then, it's not even necessarily said Golden Throne will never be used in it's original form - Magnus destiny was interwoven with Golden Throne, whereas he was originally intended to be the one that would sit on it, he was the one that were to break it, but the device is still technically intended for him - whereas I'm reluctant to believe that of redemption is possible for any traitors, he might as well be an involuntary battery (or someone completely else, but still Magnus... it's complicated), so to speak. Of course, it first would require getting Emperor's ass off the thing, which again, is much more hinted at in recent lore - to tie with the previously mentioned Guilliman talk, Emperor is pretty fucking sure Guilliman is a key to releasing Emperor from his "prison", though it's not clear as to how.
So yeah. If you move the goal posts from his original designs to "humanity gets to keep breathing", then maybe you can say humanity can "win" that fight.
That is his original design, heck, quite literally "design" if we take the shamans thing to be true. I'm not moving the goalposts, you are.
Otherwise it's just trying to hold on to what they have as, with each defeat, more is lost. In the infinite continuum of a 40k setting that never progresses, that state can go on forever. But if the setting has to advance, by its own construction, humanity will continue to lose more than they gain with each passing year until eventually the Imperium collapses.
By it's own construction, Warhammer always existed in a state of perpetual "we'll get you next year" for every faction. The point is that every faction (and many subfactions) have pretty clearly defined shot at "winning" by achieving supremacy (ar at least getting significantly ahead) over others, even if it's more or less likely depending on who you ask.
One theory as to why Chaos is so damn wack, is because the rest of the universe is so damn wack. There are probably benevolent chaos gods in the warp. In fact, John Blanche once said in an interview that he beloved that there were infinite, all waiting to bubble to the surface. The reason we don’t see Chaos Gods of like, I dunno, charity, is probably because there’s isn’t a whole lot of that going around. The galaxy right now is pretty fucked up. Even without Chaos stuff wouldn’t improve THAT much. And don’t say Nurgle is the god of charity. He is in meme-form, but not really canonically.
I'm pretty sure it's more dualistic. It's canon that technically all Chaos Gods have "positive" aspects, they're just outweighed by the negative ones. This doesn't necessarily mean that say, Nurgle, as a individual being that sits somewhere in the Warp would (somehow) suddenly become nice, but rather he could be replaced by one of his aspects that exist in lesser form up to that point. A daemon, if you will.
So, Big E COULD technically “defeat” chaos. He’d have to create peace across the entire galaxy, maybe even the universe, and at the LEAST that would make Chaos stop being so Schizophrenic. Probably. Maybe. Hopefully
Cabal saw it could happen in two ways - Emperor dies, and Chaos is destroyed simply because Horus goes on a murderous rampage to fucking kill everything, so there's nothing to feed Chaos emotions anymore, so it would starve. This was the "good" option for them.
The other option was Emperor winning the fight, and making the humanity dominant species in the Galaxy, which ultimately would result in Chaos winning because even Emperor couldn't deal with that. This is the "bad" option, and often assumed to be the canonical outcome, since, well, technically Horus did lose. I believe it's more complicated than that - as the "choice", so to speak, was up to the Twins, Alpharius and Omegon. Supposedly they're both heretics, but some (nearly all of them) of their actions do make you question their actual allegiances, and while the whole "Omegon is Janus" theory has been disproven (though Ianius itself is a pretty fucking interesting, especially in relation to Magnus and curiously, Blood Magpies), I don't think it's too big of a leap to think that somehow, due to them being... in the weird state of being twins, despite not being supposed to, they could have disagreed which caused a third option, and that one resulted in essentially a stalemate, and future is uncertain.
Speaking of which, what do you guys think is going on the next galaxy over in 40k? I wonder if Andromeda has Orks infesting it yet. I know in the fluff some techpriests launched a probe that passed the border of the galaxy like 10,000 years ago, and they’re still hearing Ork signals.
Orks. Tyranids. Lost Legions. Some Jokaeros who were particularly bored. One very confused Guardsman.