But surely having a good writer and a good director would give you just as much chance? Most of the "best" movies of all time were written and directed by separate people.
Again: In Hollywood movies, regardless of what the credits say, the director always makes the ultimate decisions about what's in the script. Usually the director is guiding the scriptwriting process, making the decisions about story and character, or at least approving or rejecting the screenwriters' ideas. So you can never truly say that the director is "separate" from the scripting process. Often the director is the only constant in the process while various screenwriters and script doctors, mostly uncredited, come and go and work on various drafts whose fragments are eventually stitched together Frankenstein-style into something the director is satisfied with.
You can take a script by the most brilliant writer around, give it to a director who doesn't value the written word, and see it turned into a piece of incoherent hackery. For example, compare Joss Whedon's original script for the Buffy the Vampire Slayer
movie with the disaster its director turned it into. Writers in the Hollywood feature film industry have zero power to protect the integrity of their scripts unless
they're also producing and/or directing them. That's why I don't trust directors who aren't also writers to make well-plotted movies.