dear rabbi kornfeld shlita yasher koach one of my talmidim asked if years cannot be taken away as per "dor holaych v'dor bah "how can years be added to the talmid chacham ?
rabbi elozer kanner
Your question is that since Hash-m does not remove a generation until the new generation arrives, so too he does not let the old generation remain after the arrival of the new generation. How can a Talmid Chacham remain when the new generation arrives?
The Rambam in his introduction to the commentary on the Mishnah writes that prophecy "for evil" i.e. bad consequences can be withdrawn. Prophecy "for the good" will always happen.
Here too we can say that "Dor Holech" includes two assertions: one is for the good - a generation will not go until a new generation comes. The second is for bad: a generation must leave before the new one comes. In light of the Rambam we can suggest that the first, which is for the good, must happen. Consequently the Gemara asks, "How could Miriam die before her time?" The second aspect, being that it is "for the bad", does not have to happen. Sometimes Hash-m is sympathetic, so to speak, and does not do "the bad"; consequently, the generation that is to be removed lives longer than its allotted time.
Mordecai Kornfeld