THAT IS MY COMPLAINT. GW HAS DONE FUCK ALL WITH IT.
All we get from Chaos stories is that some Chaos army rocks up to some Imperium world, or that some Chaos corruption happens, but no matter the outcome, it does nothing to dent the Imperium, it doesn't change the status quo, and Chaos might as well be a fart in the fucking wind.
Everything that you said would have been worthwhile, if it wasn't for the fact that Chaos, the thing that broke the Imperium during the Heresy, the force that kept the Emperor up at night, the main reason he even banned religion in the first place.........is relegated to the same position Team Rocket has in the Pokemon anime. They're punching bags. Chumps. They can't even get their shit together, and all they do is fight each other or get Rowboat's boot up their asses. Four gods, more demigods, more advanced tech, and war veterans whose battle experience stretches millennia, and they're about as threatening as Team Rocket trying to steal Ash's Pikachu for the umpteenth fucking time. It doesn't matter how many people they corrupt, it doesn't matter how far their influence spreads, they're just brainless animals fighting each other and barely even threatening the outside world.
You do realize that GW has done fuck all with the "plot" of 40k for almost 20 years(basically 3rd edition up until the start of 8th), right? Even with just the Eisenhorn stuff it took them 9 years to get from Bequin Pariah, to Penitent. It took 6 years for them to eventually put together a novel about the fall of cadia. Hell, if you go back to the original eye of terror campaign thing, that makes it 14 years to get resolved, and still 20 to end up as an actual book that wasn't just bullshit lore blurbs attached to rules.
With regard to how utterly glacial GW has been when it comes to moving shit along for 40k, of course they haven't done a damned thing with Chaos, especially "winning" in any meaningful way. It's why they had to make tyranids the latest big bad with 10th edition because an army of primaris marines and the indomitus crusade need to be distracted by something stupid so they couldn't just keep going along in the Dawn of Fire series and score a bunch of victories.
Genefather: Fabius Bile wins. But that doesn't mean much since he just got some material for his "nu-men" that won't amount to shit.
Arks of Omen: Vashtorr and Abaddon were allowed to get the parts they needed for their whatever key thing, and have done fuck all with it. At least Angron technically got a win by breaking the choral engine that was supposed to have been a small equivalent of the beacon on terra.
Ahriman series: He's been busy fucking around with xenos, still trying to undo the rubric.
Ephrael Stern: Apparently before The Heretic Saint she was on the opposite side of the rift and was travelling back to the imperium controlled side, and we hear nothing about the other side than it's a total shit show. I don't recall what Kyganil fucked off to do by himself.
Dawn of Fire series: The past few books have basically been about girlyman ordering some ships to find a way through the rift via some passage. In the latest one, Sea of Souls I think the name is?(it's the one with dollar store eddie murphy on the cover) Chaos manages to take over... 1 ship. The damned Black Templars in an earlier book took out a damned torchbearer fleet because one of their chaplains was afraid of primaris, so technically BT got more done on accident than Chaos.
Bequin: These books don't even take place in the correct millenium to be moving anything along at this point.
Dark Imperium: Chaos was winning... until the emperor had to make robot girlyman into a super saiyan because otherwise there was no way he was winning that fight, but it wasn't like they were going to just immediately kill gorillaman after bringing him back into the setting. This was just shit writing all around as they had written themselves into a corner
The Lion: He's fucking around gathering up forces at the end of the sons of the forest book to fight against chaos in the rift, but of course the book isn't focused on that at all and he takes out the leader of a chaos war band, whoopdeedoo. Lost 1 or 2 planets in the process.
So yeah, Chaos can't ever do anything major because Chaos can't simply and will never "win", as their wins already happened with the scattering of the primarchs and the horus heresy itself. GW is never going to change this. And while Chaos is busy not winning anything, at the same time GW also still has other factions they need to have people writing for because Chaos isn't the only antagonist in the setting. The Indomitus novel, as awful as it was, the imperium lost to necrons. I don't know what you're expecting Chaos to ever be doing in the setting other than amounting to a foil and punching bag for the imperium, but that's literally how it's always been.
Again, if they were this non-threatening, the Emperor keeping them a secret from his sons was an even bigger goof. He could've just told them the truth.
I don't think anyone argues that the Emperor wasn't an idiot. Remember he had been alive on Terra for thousands of years, implied to have even been Alexander The Great, so that's basically 30,000 years of the emperor failing one way or another.