Screenwriters Mark Purvis and Robert Wade have been responsible for both the dumbest Bond screenplay and one of the smartest, and after five films they've decided it's time to move on from the franchise. The duo wrote screenplays for all of Daniel Craig's films along with The World is Not Enough and Die Another Die. While we can't blame them for Quantum of Solace's issues since they were on strike at the time we can blame them for pretty much all of Die Another Day (though the screenplay deserves a bit more credit than it gets).
At the fourth Doha Tribeca Film Festival Wade explained they were leaving and revealed that Sam Mendes and Skyfall's other screenwriter John Logan had an idea for the next film. “We’re very happy to have done five Bond movies, I think we’ve gotten it to a good place. I know that John Logan and Sam Mendes have come up with a plot for another one, which takes the pressure off because these films take up a lot of time.”
Mendes isn't confirmed for the next Bond yet, but I don't think anyone would be too upset with him coming back. As for Purvis and Wade l'm not too upset to see them leave. Their two best screenplays (Casino Royale and Skyfall) were both re-written by others to get them to perfection and I think with the ending of Skyfall being what it was it's a fine time to get some fresh ideas in.