Remember that Brian Reed wasn't the one at fault with Halo 5's story. Following the fan backlash of Halo 4, Microsoft was very restrictive, pulling their weight over 343i way too much and creating the abysmal story we got now. Microsoft wanted a Halo story that was more accessible to commercial gamers, and so the cliche Created story was used, which obviously in the Halo setting, and especially post-Halo 4 story, it was jarring and... terrible.
There was only so much Brian Reed could do with Microsoft's strict and awful 'guidelines'.
Even if Eric Nylund, through some cosmic alignment of the stars, became narrative director, that doesn't give him creative control to make a masterpiece. The story is written by several people, and people like level designers and those who work with the engine also have a lot of input. There are a lot of working pieces, narrative director is only one of them.