If something of the kind occurs during the negotiation then everything would be dashed to the ground once again and even bigger war will start among the groups who have been fighting for centuries. A dark force has other plans and it does not care what Harry thinks about the future of the supernatural world and the world of humans. Harry is the only one who holds both world dear and he just cannot choose anyone of them.
The dark force tries to destroy Chicago and everything which exists in it this means that Harry has to end that darkness first. James Marsters has narrated Turn Coat and The Law too so the audience likes his voice in this series and no one at this point wants a change in narration. Thus it is better that the narrator goes on with the work till the end of the story.