You never blame a writer for the final product. The script is at the mercy of directors, actors, executives, financiers, marketing, editors, costume designers, gaffers, sound design, color correction, music and, most of all: Reality.
The Darkest Hour was a rewrite job for Spaihts, and I believe it came post-Prometheus. Damon's movie's make money. He's very in demand, and has even had to turn down high profile jobs including a Marvel movie. Think it was an Iron Man movie. Well, both of those guys are in demand.
Blame Fox or Ridley if you didn't like the movie, don't blame the writers.