Like Moe Greene later on down the line, there would be the inevitable interference from another family. Conflict would have been inevitable, with the enormous revenue potential of narcotics.
If you think about it, from a strategic standpoint the shooting of Vito worked out better in the long run. Sonny overplayed his hand and was dealt with, and Michael was pulled from the outside into the eventual head of the family. Had that not happened, an older but still rash and impulsive Sonny would have assumed control in the mid-1950s and the family likely would have collapsed on itself.
I agree that regardless of Vito's decision,Sonny was the fly in the oinment.
No matter what,Sonny as the head of the Family is a disaster in the making.
It is possible that if the Corleones went along with Solozzo and Sonny succeeded Vito,that Mike would have still become the Boss. I say this because it's almost a forgone conclusion that Sonny would have still gone to war with some or all of the other Families,whether it be over drug profits,or some other reason. I see Mike as having to step in just like he did,in order to "protect his family".
