According to the novel, I'd agree with Peter.

Puzo wrote something to the effect that the position of Don "was by no means hereditary."

There was no way that Fredo was ever gonna hold the reins.

Had Sonny lived, I think that there's a fair chance - despite all of the "every man has but one true destiny" stuff - that Michael would have chosen to leave a life of crime completely. Had he stayed, I believe he almost surely would have deferred to Sonny as Don.

But he certainly would have been the #2 man, and the "behind the scenes brain" of the family, providing the intelligent reasonableness of his father as a counterpoint to Sonny's temper.

I don't think that Sonny had the brains for the big deals that Michael did either, nor did he have the long-term vision.

Actually, they might have made a very effective "Two-Headed Don", with Sonny as the outside man who handled the muscle end and keeping everyone in line, with Michael working behind the scenes by negotiating, making the money, and eventually legitimizing the family.