Halo actor Steve Downes says that he challenged the direction that 343i was going with the Master Chief in the advertisements for Halo 5 Guardians.

Instead they brought in a new Spartan warrior from nowhere, called Locke, whom we were suddenly expected to cheer on as he hunted down Master Chief, who had instead gone off on his own adventures. But ...

Steve Downes, the voice of Master Chief from the Halo games, goes on the record about AI voices replacing actors.