If Sonny was alive, the other families would not have thought that the Corleone family was weak......and Sonny probably could not have wiped them out like Mikey and Don VIto did........Sonny was too hot headed to play weak.
But, with that said, you are all right......Sonny would have run the family into the ground, Mikey would have gotten assasinated, and Fredo would have been iced out in Vegas......the family would be no more.