Having jumped ahead to the future, they would have advanced their own death date, allowing them to work on something to save all the passengers.
Is Manifest Based on A True Story? The question is, what is it exactly? This put his role on Manifest Season 4 in jeopardy, but Long has since been saying that he will return for some of the season though his role in the show has been less impactful, so he might be making the correct decision by reducing his screen time.
As for why the Earth needs almost 200 prophets, that's less clear.