: They said they always intended that to happen, and you're first reaction is
: to just say they're flat out lying?

Yeah. It's patently obvious that the Reclaimer Saga went through extensive rewrites after Halo 4. Remember the GDC panels where the writers were talking up the Didact as the Chief's big nemesis? The Didact was supposed to be the new villain. He was clearly not supposed to be done away with after one game and three comics.

The Next 72 Hours was clearly planned prior to Halo 4; notice that the Halo 4 legendary symbol resembles a Forerunner skull with an eye stabbed out, as happened with the Didact. Notice that the Didact's fate in both Halo 4 and TN72H plausibly leaves him alive as some sort of AI, a rare dovetailing of game and expanded universe that actually works well.

And in Halo 5... where's the big bad nemesis?

Same can be said for some other plot points 343i was hinting at. At one point, the Chief was supposed to be going on a journey and finding himself after the loss of Cortana, or something like that. Then there's Poncho Chief that the writers and artists were in love with, but couldn't make it fit into the game.

I don't think 343i knew where to go after Halo 4. Certainly, they had tons of ideas and a general direction. But the lukewarm reception to the Didact, Majestic team, and Spartan Ops in general probably led to a shift in storytelling.

: ): I actually think it makes perfect sense. Many (most?) people suspected
: Cortana wasn't really dead, so that's no surprise.

Cortana could have turned evil any time in the past 15 years, and many people would have shouted "Aha! Saw it coming!"

: Sure, her actions are
: drastically out of character, but that's intentional. I don't think it was
: just some hamfisted "Let's make Cortana bad!" shoehorned into
: the story.

Why not? The Chief going rogue was.

: They have (and have always had) a plan for this, the entirety
: of Halo 5 is just the opening.

Three years ago people were saying that Halo 4 was just the opening to 343i's grand story. People said that Spartan Ops was just setting the groundwork for something larger.

It's time to admit that what's being released right now is the meat of the story. This is it.