Karen Traviss shouldn't be allowed within 100yards of a sci-fi universe
Truer words have never been spoken.
Gears 3 displayed similar symptoms when she went from writing just companion pieces in the expanded universe, to penning a trilogy of novel sequels to Gears 2... and then ending up as main writer for Gears 3 as well. So naturally the third game was made with the assumption that you've read the novels, and players who hadn't kept up would feel as if they've somehow missed a whole game or two worth of story.
Both the hardcore and the casuals abandoned Halo 4. The numbers as whole took a nose dive because Halo 4 wasn't Halo. I have serious doubts about 343 being able to recover with Halo 5. I'm positive initial sales will be really strong as always, but can it sustain itself like a Bungie-made Halo game? I doubt it very much.
You're right, Halo had managed to retain an audience of its own - both hardcore and casual - up to 4's release, because the series offered an experience you couldn't find elsewhere.
And that's being said as someone who didn't dislike the gameplay changes at all; I enjoyed my default sprint (which was secretly the best AA in Reach), my custom loadout (DMR/PP/PG) accounted for every combat scenario, and my beloved Autosentry allowed me to get an advantage over everyone else in straight fights.
But then again, I'm not the diehard fan (slash consumerist) willing to pay for online multiplayer access either. Nor was I ever part of those who thrived in the original games' online modes. They drove away hordes of loyal, paying fans in order to attract a much smaller and less interested audience (significantly more fickle for the most part too).
So 343i decides to look at the complaints fans had about H4, and thusly they decide to take away the new parts that did work (Loadouts), the parts which should rightfully be contained to their own playlists if nothing else (Armor Abilities), but then somehow they ended up taking the remaining core even further in CoD's direction with ADS, boost and unlimited sprint for everyone?
And with the reveal of the H2A multiplayer it almost seemed like they finally understood their fans too...
It crashed long before 4. Think back to Reach, if you want to really think about it. Reach set 4 up for failure
In practice, you're right; 343i took what Bungie last did with Halo's mechanics and went too far with it for most fans. In theory, even though Reach was the de facto fourth Halo game, it was still subtitled and thusly held to different expectations... and despite even those, fan reaction to Reach already wasn't as universally positive as it was to H3, most of all because of Armor Abilities. I personally didn't dislike the gameplay changes there at all, but I never expected they would be taken up to eleven in the next game.