: Why might they want to do that though?

Because they want to remove S117 and step out of Bungie's shadow. From the look of this story line and the game play, I think could do it. 343i bought the Haloverse, but as long as the Chief is the main character, they are still playing with Bungie's toys. They need to prove they can tell a story without reliance on this epic hero.

If the Chief falls, I fully expect that the rest of Blue Team will have died already. Probably even before that confrontation with Locke we saw in the trailer. And as much as it would kill me, I wouldn't be surprised if Locke fails to save Buck a third time.

I don't think Locke is a very empathic hero. That gives more opportunity for character growth, but I think that's a stretch.

Just my two cents.