It's written poorly, with huge gaps in fluff logic (Black Templars and an Inquisitor taking help from the Eldar for example). Also it looks too much like a promotional pamphlet for their new minis, than an actual, engaging story, which would make people care for. At least in my opinion.
So are you referring to the 40K fluff being poor before this story advancement, regarding this story advancement, or both?
And where is the new fluff coming from/located or whatever?
Well I mean 40K's fluff, in general, has been going downhill for some time, in my opinion of course. It's written in either a manner that is too epic, or downright ludicrous. GW has shown that while they don't give a crap about Chaos in the actual game, in the fluff it's the big winner. Look at the End Times. Nothing that heroes could do, would help them. Archaon literally won because he won, because Chaos is awesome and edgy. Same with 40K. Abaddon leads the 13th Black Crusade and there is nothing that the defenders can do to stop him. The Emperor has been consistenly shown as a brain-dead moron, who bit more than he could chew and now he's paying the price. The Imperium never could've won, in fact no other faction, aside from Chaos of course, could have won.
Prominent BL authors, like Aaron Dembski-Bowden reinforce that claim, by outright stating that "lol Imperium be dead son". At the end of the "Master of Mankind", a very good book by the way, ADB states that the Imperium is coming to an end in a manner, that really leaves no illusions as to what he means.
So my question is: why should anyone, other than Chaos players, actually care for their game, unless they're simply playing it for fun or competitvely and second question is: why is GW so damn schizophrenic. They're stating that Chaos will win, but the current Chaos meta is horrible. Also their current business and PR model, while admittedly much better, than the one from few years back, literally leaves many factions in the dust. Tyranids and Orks are borderline unplayable. The Guard is viable only, if you either take mass tanks or ally yourself with the Inquisition. The Dark Eldar are medicore to the extreme. Meanwhile we get endless updates for Space Marines, turning them into a riddicolously OP army (grav weapon spam, never fun) or CSM, but their updates blow big time. The Traitor Legions book only really made Alpha Legion playable, but factions like the Word Bearers and Death Guard got the shaft.
Overall the current 40K, both in meta and fluff is just too overblown and over the top. I really hope that the 8th edition will trim the excessive rules system and make it more friendly, for every faction, not just the posterboys or the Eldar. It's no fun playing against an army that you cannot and will not defeat, no matter how hard you try. Not to mention that the current player base is full of douchebags and assholes, who gloat, are generally unfunny to play with or against and overall suck ass. Like really, when I've started 40K back in 2008, players were mostly ok. Now it's like a season for asshats and that's something that I can't stand.