Post Halo 3, the lore and backstory was overexplored, just like in the Assassin's Creed franchise.
Simple things can no longer be handwaved away as part of pure worldbuilding, things now have be narratively explained more and more, and this overexposure dissipates any sense of mystery. (i.e. whoever wanted to know what the Librarian/Diadact looked like? Were the Precursors necessary?)
On top of that, I'd argue that one of Halo's immersive worldbuilding strengths (you and a very small cast of supporting characters in a vast, empty mysterious alien construct, or a tight narrative involving specific backstory instances) is lost as the narrative universe sprawls beyond the game, and may risk drawing in out-of-game characters into advancing the narrative (like in Assassins's Creed).
I'd agree that the entire franchise needs a weedwhacker brought in to clear out some of the overgrown universe, but some things are long out of the box and cannot be returned to that state of mystery, so to speak. IMO Halo also doesn't need to be constantly pushing out entries- sometimes less is more, though the bean counters would beg to differ.
On another note, I recall a Halo 3 blog that explained the whole backstory wonderfully- not sure if this is it, but it's interesting seeing the deciphering of the lore that players can do on their own- no official depiction necessary!
Basically. The Halo 3 story left things well enough. The humans are the descendants of the Forerunners, the Covenant leaders declared war on them because they didn't want that truth leaking out, the Forerunners wanted their descendants to inherit everything they had, the Flood were the ancient enemy of your ancestors (ie. when the Flood Gravemind says "Child of my enemy, why have you come? I offer no forgiveness. The father's sins, pass to his son.")
If they wanted to push out more entries, they should have pushed out more stuff from the Human vs. Covenant War or side stories happening while the main trilogy was happening. Like say, have Halo Wars 2 happen during the Human-Covenant War, have the Spirit of Fire bump into the Banished in the ass end of space far from the UNSC as the Banished are waging war against the Covenant. The Spirit of Fire crew and the Banished start out fighting each other, but when they realize they're on the same side against the Covenant, they break bread, make peace, and work together. Or have a game taking place from the POV of an up-and-coming Sangheili Zealot as the Covenant Civil War breaks out, and you fight all over the place, you fight Brutes loyal to the Prophets, Insurrectionist humans who are using the UNSC's weakness in the wake of the Covenant invasion of Earth to attack UNSC colonies, you even fight some Sangheili warlords who hate humanity and the Arbiter in order to ensure the Sangheili-human alliance is solidified by the time Master Chief arrives on Earth.
But no, they had to keep on beating the story to death after Halo 4. Halo 4 is nice enough, but it wrapped things up well too. I wouldn't mind more stories after it, but 343's expanded lore destroyed any lingering mysteries about the Forerunners and they decimated any excitement people had for the new bad guys. The UNSC is controlled by a corrupt shadow government from ONI, the Covenant regressed back to what they were in Halo 1, the Brutes are bad guys again even though there's no Prophets to lead them to fight against humanity anymore, and for some strange reason, Cortana's the enemy even though she should be humanity's ally. It makes me want to bash my head against the wall.
The precursors were actually alluded to in Halo 3. I think they were described as "legends of the past" or something but they did exist in Bungie era Halo.
I don't mind the expanded lore. Greg Bear's Forerunner trilogy is a great read. As long as the expanded universe isn't required reading for the games going forward, I don't see the problem. Likewise with games like Halo War 2. I would expect there to be some sort of exposition to explain the origins of the Banished and whatever happened to humanity vs the created.
The Precursors weren't alluded in any previous Halo game. At most we just had the Forerunners being ancient humans, which got sledgehammered by Greg Bear's Forerunner Trilogy. 343 Guilty Spark openly tells Chief "You are Forerunner" even though the Forerunner Trilogy proved him wrong later on, which goes to show that Halo 3 intended for the Forerunners and humans to be the same species, yet later books disproved that theory so as to pad out conflict between humans and Forerunners. Halo 4 made good use of that conflict to create a decent story, but the story contradicts previous lore established by the games that clearly makes the humans the biological descendants of the Forerunners, which explained why the Covenant leaders waged war on them.
Now, for some reason, the Forerunners and the humans were rival species, but the Forerunners somehow made the humans their heirs even though they blasted the humans back into the stone age? It makes no sense, and the previous explanation was so much better-the reason why Forerunner tools and tech works for humans is because, as 343 Guilty Spark says, humans ARE Forerunners. At most, you can explain humans away as Forerunners who somehow became primitive and forgot all about their previous tech, which has happened in sci-fi before when people get stranded in a primitive planet and they go native. So it makes sense why Forerunner tools work for the humans despite the humans having forgotten things and became primitive, because they still have Forerunner DNA in them, and the Forerunner machines automatically recognize humans as Forerunners.
In my own Halo stories, I had to re-write things to return the lore to Halo 3 levels. To justify Forerunner machines working for humans and the Forerunners making them their heirs, I decided to add in that the Forerunners and humans were once the same species but drifted apart and evolved differently, with the Precursors favoring the humans, leading the Forerunners to kill them and wage war to drive out humanity out of their space in a sort of Cain and Abel scenario, which explains why the advanced humans would later invade Forerunner space instead of asking them for help when the Flood invaded. And I wrote in that the Precursors favored the humans because they were once humans themselves before their own scientific experiments and search for forbidden knowledge turned them into abominations. That way, ancient humans are still the precursor race, a theme Halo 3 beat into our skulls rather conclusively.
They already explained the origins of the Banished as Brutes who wouldn't take orders from the Prophets and got tired of being Covenant cannon fodder. As for Humanity vs. the Created, there's barely any excitement for it. It's the typical Skynet plot with a PMS-ing Cortana as our Skynet. Quite a step down from the narrative masterpiece that was Halo 2 or the epic tragedy which was Halo 4.
I'm just gonna say it; for as bad as 343 are (very!), it was really Halo: Reach that killed the franchise. It was clear Bungie didn't care anymore and was just throwing shit at the wall to see what sticks. Unfortunately a lot stuck and it still reeks to this day.
343 could turn things around if they had a passionate team, but employees were literally hired for their passion of hating Halo. So that's not happening. I feel bad for Staten, he's in an unwinnable situation and will inevitably take some of the blame.
No it didn't. Halo Reach was well-loved during its time, and people played it all the time and loved Bungie for it.
As for 343's people hating Halo, that's the same case with Star Wars where the current Story Group and the people in charge hate everything that the SW Original Trilogy stood for and did their best to undo those movies with the Sequel Trilogy.