If the show had failed, CE would have been happy to have said that he wanted to move on. Now after the 10 million viewers tuned in, the first series is complete he decides to call it a day.
Jane Tranter Head of Commissioning drama confirmed yesterday that his contract included an option to do further series. I think CE is seeing pound signs before his eyes. "Sell" your story to the Scum, more publicity for the show and turn around to the BBC and say "I will do another series but only for £xxx,xxx fee."
Personally I wouldnt mind a new Doctor especially if the regeneration were to take place in a Christmas Special. If I were RTD I would love to do a 2 part story and dump CE at the end of the first part, giving the viewers a chance to get to know the new Doctor before the second series starts.