Actually, the Ancient One has occasionally manifested and communicated to living folks (mostly Strange) since his death. The circumstances of that were... unique in human history... and left him conscious, capable of action, and actually one with all of reality. So, in a better position to judge than a senile former holder of the office who spends his days levitating and drooling.
Back in 1974 the Ancient One said that he was bequeathing his mystical powers to Strange (hence the latter being SO MUCH more powerful than other mystics and able to fight Dormammu, Mephisto, etc. on an even playing field). It now looks as if maybe he was actively bolstering Strange from within all along, withdrew that help when Strange lost his confidence and decided to abdicate, and is now supercharging Jericho Drumm instead. Which means we're not quite so screwed as if Strange disappeared and left someone whose most impressive magic was card tricks to defend the planet untutored.