So everybody was equally invested story-wise in all three films. So the guy that got the third movie, which is the middle of this new trilogy, he now knows what preceded and what follows out of what he's writing at any given moment. We all consider that to be a really exciting, creative and groundbreaking experiment in screenwriting. It worked as a process to get our minds around this epic and all these new creatures and environments and characters.
Imagine, if you will, that instead of screenwriters crafting movies you had political parties crafting public policy. These parties know they're going to be authors of some policy pieces or leading the process at one given moment of time or another, but aren't quite sure which during the planning phase.
While each party brings a different perspective to the table, they all know and believe in the bigger picture they're contributing to. They're passionate about getting it right, because they know what an important story they cal tell - what a positive impact they can have.
... or as Cameron has done: plan really far ahead, accounting for countless details and, naturally, leaving lots of notes.
The goal couldn't be to get ahead as quickly as possible, leaving others behind; it would have to be the other thing.