5. New Mouth, New Rules
The main reason why Steven Moffat's smart choice would be to walk away now is simply because his Doctor is leaving. With Moffat's brain and Matt's heart(s), these two have created a whimsical, brilliant, angry, ageless man that is remarkably easy to love. At the moment, only half of Eleven is walking away, and while Moffat has proven he can handle writing for different Doctors, his continued presence could present a problem. Namely, if he leaves in the middle of 12's run we will hit a very uncomfortable snag in the fabric of the show as the style, language and dynamics change while the actors stick around. It would be more prudent for him to bow out now alongside Matt Smith and let someone else fly the TARDIS alongside Peter Capaldi.