There is speculation that Russell T Davies will leave Dr Who after the fourth season, which I could totally understand.
Someone who gets into that kind of job isn’t the do-one-thing-until-you-die type, especially when you consider the topics he’s done before. I imagine the restrictions of a 7 pm show might chafe after a while, for the guy who wrote “Queer as Folk”!
After the bloom is off the rose I could see any series becoming a drag, middle aged.
The beauty of it is he can always come back to guest write an episode.