If we're entirely honest with ourselves, old Billy's departure at that point was the best for the show at the time. Obviously John Wiles didn't last much longer as executive producer and perhaps a new producer wouldn't have considered the concept of regeneration, bringing the show we all know and love to an untimely end. Plus, you have to consider that Hartnell's health was prevalent over any desires to continue as the Doctor, and he wasn't a well man by any means at this stage.