: The Librarian's plan is what she called it—an eventuality. Humans would
: eventually come up with power armor and AI assistants and all the tech
: stuff they once had, and the Librarian wanted them to inherit all the
: Forerunner's gadgets (she never called Master Chief into existence). But
: she didn't want them to inherit the Mantle, which she held as one of the
: reasons the entire galaxy fell to the Flood (this goes all the way back to
: Halo 3).

: The Forerunners held that the Mantle is basically a responsibility awarded to
: the top dogs of the universe (them) and they used it as justification to
: suppress anyone else from reaching their level of power. That's what the
: Didact is going on about, although he couches it in your usual "I'm
: doing this for your own benefit." Cortana and the Warden are saying
: that AIs are the inheritors of that legacy, not frail meatbag humans. The
: whole painting their assumption of the Mantle as "the real plan"
: is just wrapping their regime up in some trappings of legitimacy.
: "Believe me, if the Forerunners were here, they'd totally agree with
: us on this one. So be quiet or I'll send my space bird to nuke your
: technology."

There's also a great distinction between the Mantle as the Forerunners saw and assumed responsibility for it and the original intention as set out by the Precursors (the ones who actually initiated the concept in the first place). The Created may be the more logical choice to inherit the Mantle as it was seen by the Forerunners, but Humanity were the next in line to be tested as set forth by the Precursors. Whether Humanity deserves it either remains to be seen, but...yeah.