Re: 7X13 The Name Of The Doctor(Grading/Discussion)(SPOILERS!)
The new regenerations for the Master required the involvement of the high council of the Time Lords. Since the reformed Master catapulted them back into their isolation, the writers could always one day let him return as Rassilon's replacement as a leader and kill two birds with one stone. They'd only need a new actor, since Simm's version mentioned that the body was bound to die one way or the other.
Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning.