Like Fredo, I think Sonny was simply incapable of growing beyond a certain point. Sure, a recuperating Vito would certainly have tried to guide him and sure, Sonny would not have defied his father but still, in the long run he could never have become an decent Don or even anything close. And all of this would've kept the story effectively dead-in-the-water until Sonny's death, which was an eventuality no matter what. Michael would've had to remain in Italy far longer than he did. He might have enjoyed a longer marriage with Appolonia, but still that leave the story hanging. Sonny's killing by Barzini (and Puzo) was specifically planned in order to get things moving. I guess it's fun to speculate what might have happened if he'd lived a year or so longer, but in terms of the actual storyline, he died exactly when he was supposed to.

Apple