We do have proof that lives can be extended. In "The Five Doctors", the Master (who by that point had already had 1-3 incarnations after his supposed final "natural" incarnation, depending on how you look at it) was offered another regeneration and a completely new life cycle by the high council. He accepted this as though it weren't a big deal.
I always just assumed that the council was basically BSing the Master. They may have promised him a new life cycle, but that doesn't mean they could have actually given him one. (And there's no real proof that the Master even believed them when they said that.)