Why? They could no longer hold on to the 1914 + a human lifetime and they could not come up with anything more imaginative.
I once thought, before this change, that they would simply extend the generation to either the oldest person alive (currently 116 years old - https://en.wikipedia.org/wiki/Susannah_Mushatt_Jones) who was born on or before 1914 or the account in Genesis about 120 years being the time limit before the flood.
So the overlapping generations not only shows how desperate they are but how unimaginative they've become. If Freddie the wizard were alive today he would have cooked something up much better than the overlapping generations.
By the way, has anyone on this forum calculated the date in which this overlapping generation is limited to before it becomes falsified or can you have a third overlapping generation?