I agree, like Randy Pitchford commented on the making of brothers in arms, he mentioned that in Titanic, people walked into that knowing that the boat was going to sink, and they wanted to see the special effects, but the story wasnt primarily about that, it was about this amazing drama between two characters. A STORY that we didnt previously know.
This (i am legend) is strange in that, as stated, we already know everything, the drama, the story, what happened previously, how it got that way, they would just be retelling the same story again with more detail, which is totally unneccesary IMHO