In his statement, Affleck suggested he was stepping down from the role out of respect for the character, and he had found it difficult to dedicate the necessary focus required to serve as writer, director, and actor. A Warner Bros statement supported Affleck's move.
Countless theories have emerged trying to figure out why the recent events have unfolded. One of them suggests that Affleck and Warner Bros had never been on the same page concerning the script. At various times, it was either incomplete, not properly done or not ready for filming, depending who was asked at different stages.
Another possibility for the script was because Warner Bros were enthusiastic to release the film on an individual schedule, while Affleck wanted to take his time to deliver a perfect script and did not want to commit to a release date.
Affleck will remain as the Executive Director, though, and it's now up to Warner Bros to look for a replacement Director.
Affleck's script will most probably still be used with a few rewrites here and there which are inevitable. But it is highly unlikely that the entire script will be discarded altogether.
Also, by stepping down as Director Affleck did not mention anywhere in his statement that he would not be starring in the much-awaited blockbuster.