Sonny's death, without it, Michael never takes over.
It does make me wonder though... Michael saw Sonny in action as the acting Don... he was concerned.... he saw that Sonny was a bad Don as Vito said later. Could there have been a sibling battle over Donship at some point if Sonny lived and constantly did things wrong and ran the family in the ground... so to speak...
"What I want.... whats most important to me... is that I have a guarantee" - Train approaches.... Bang! Bang! Bang!
