Mark Twain famously wrote:
The Egyptian, the Babylonian, and the Persian rose, filled the planet with sound and splendor, then . . . passed away. The Greek and the Roman followed. The Jew saw them all, beat them all, and is now what he always was, exhibiting no decadence, no infirmities of age, no weakening of his parts. … All things are mortal but the Jew; all other forces pass, but he remains. What is the secret of his immortality?
That answer is not provided by the normal rules of historical causality, despite the best efforts of superb historians to discover it. It is a riddle. (Appropriately, my friend, Dr. Simcha Goldin subtitled his book about the medieval Jewish community 'the riddle of Jewish Survival in the MIddle Ages').
The only answer I can suggest is developed by Ramban, and provides the basis for R. Meir Simcha's comment. When Abraham was chosen by God, he effectively left the realm of normal causality and entered into a different dimension of history and time (Historia Sacra, as Krochmal termed it). It was that deeper reality that was embodied by Moses' birth and death; a reality that Haman missed and that makes Adar-Nissan emblematic of Jewish survival.
As Ramban (Ex. 6, 2) points out, whether Jews stay in that sublime state depends upon their behavior; whether they live up to the dictates of the covenant that Moses negotiated on our behalf.
Now, there's the rub.