I don't know if it is essential in this arguement, but I remember the Godfather novel mentioning that it wasn't hereditory in the Corleone Family either....

It goes on to argue that Mike was better prepared and a better potential Don than Tom, Clemenza, or Tessio....

I haven't seen the Sopranos, so I can't comment on the general topic, but for whatever it's worth, I have read on numerous occasions that passage of powers was by no means hereditory (in several cases, it resulted in "civil war" among the factions in the Family that does it...)