The words of producer Philip Hinchcliffe: "We tried to get famous actors for the faces of the Doctor. But because no one would volunteer, we had to use backroom boys. And it is true to say that I attempted to imply that William Hartnell was not the first Doctor."
Chiball didn't do anything that Philip Hinchcliffe didn't do almost 50 years ago; and, in 'The War Games' the Second Doctor said that Time Lords have can live forever, barring accidents.
Then there's this exchange between the Fourth Doctor and Runcible from 'The Deadly Assassin'.
DOCTOR: Runcible, my dear chap. How nice to see you.
RUNCIBLE: What? Oh, I don't believe we've, er. Oh, I say. Weren't you expelled or something? Some scandal?
DOCTOR: Oh, it's all been forgotten about now, old boy.
RUNCIBLE: Oh, really? Well, where've you been all these years?
DOCTOR: Oh, here and there, you know. Round and about.
(The Doctor bends over to keep his face hidden from the searching guards.)
RUNCIBLE: Is there something the matter?
DOCTOR: Oh no, just a twinge in the knee.
RUNCIBLE: Well, if you will lead such a rackety life. Have you had a facelift?
DOCTOR: Several, so far.
Robert Holmes co-wrote 'The Brain of Morbius' and wrote 'The Deadly Assassin'. Had Producer Phillip Hinchcliffe and Script Editor Robert Holmes not left at the end of the fourteenth season, who knows if they would have explored this avenue further.
Deadly Assassin introduced the 12 regen limit.
Mawdryn Undead established how many the Doctor had used.
The Three Doctors and Five Doctors does much the same.
And as a bonus, Deadly Assassin introduced the Matrix, which is established as infiltrated by the Master and used to create a lie in its very first appearance.
It’s rather easy to play tennis on this one, because it was rather poorly done. And gets repetitive.
RTD2 is going to do something with it, the question is, will it be any good? Do we have faith?
Oh the irony.
And if we’re going to worry about what producers would or wouldn’t have done if they had stayed, then do we also count Ulysses? Borusa in the Tardis? Cybs? The Master is the Doctors brother? The Doctor died, and the temple of the fourth, as Ace becomes a Time Lord?