Are we there yet? If I was going on a journey of great proportions, I wouldn't be taking along an infant race.
Paolo Valentino (firstname.lastname@example.org) writes:
Try to imagine that there was a relationship between the Prophets and Forerunners, "complete devotion to the good masters" and "care for the primitive pupils"
The Forerunners discover this young and inexperienced race, the Prophets, who then worship the Forerunners as Gods. The Forerunners continue with their own affairs, maybe in a portion of space far from Prophets' homeworld, and are forced to fire the Halos, leaving the Prophets behind
In despair, (perhaps the Prophets are not fully mature, or even regarded as sentient) they began to think that their Gods have gone away, leaving them all the technology and knowledge necessary to follow them.
The Prophets are trying to activate the Halos to join their Gods on their Great Journey. Meanwhile, they develop the Covenant, converting the other races to their new religion. With the aid of their Masters' technology they have the same effect on Grunts, Elites that the Forerunners had on the Prophets.
Then, it make sense that "this time, no one will be left behind".
I had to do some editing, since English is not this poster's first language. Interesting theory, hinging on the thought that the Halos only destroy sentient life.