I honestly think that Michael would have stepped aside for Sonny, being the eldest. He probably would have operated as Sonny's Underboss (much as Sonny did for Vito), and possibly Consiglieri (since we all know how Tom was as a wartime consiglieri...) or been the "legitimate front" of the family like what happened to Tom when he was "out".

If Sonny continued to live, why should Michael get more involved in the crime business at all?

I agree D. We can only guess at what point Michael committed to a life of crime. He murdered Sollozzo to protect his father. After that, who knows why.

Right there, Oli, is the most fundamental question in the Trilogy.

In the novel, Vito said (of Sonny): "A man has but one destiny." And Sonny wanted to be his father's successor. But Michael wanted nothing to do with the family until Vito was shot--and Vito didn't want him involved (at least directly) in the family business. And, Michael had alternatives to every criminal decision he made.