Uplifted to succeed the Forerunner, maybe. The Precursors have been around for a hundred and fifty billion years, so I doubt they were stepping down anytime soon.

: the big thing to me that happened between the creation of the forerunners and
: the uplifting of humanity was the uprising of the forerunners and the
: toppling of the Precursor regime.

So, if the Forerunner killed the Precursors at Path Kethona before Humanity could be uplifted... how did Humanity build an interstellar empire in the first place?

: I much prefer them being separate and distinct life-forms

Problem with that is that the Librarian is a fangirl for Humanity for... um... no good reason, really. If Ancient Humanity and Forerunner are two flawed halves of a whole and Modern Humanity is the offspring of both, then that means that our actions can redeem the sins of our fathers.

It's a lot more compelling to me than "Humans are just more awesome/creative/intelligent/varied/diplomatic/tenacious/procreative than other species."