It's true that in the novel, the Don said, "every man has but one destiny"--but he said it about Sonny, after Sonny told him he'd seen Vito kill Fanucci. But Michael? I think that if Sonny had lived, Michael could have returned to "civilian" life and even become the senator or governor his father wanted him to be. The novel shows how the Bocchicchio family provided an iron-clad way for Michael to come home from Sicily and to escape arrest for the McCluskey/Sollozzo murders. Assuming he could have forsworn vengeance for Apollonia (or postponed it for an opportune time), I think that Michael would have welcomed the opportunity to return to college, marry Kay and live a "normal life." Sonny, IMO, was destined to die by the sword. If he'd survived long enough for Michael to have returned to civilian life, and died afterward, it would have been very interesting to see if Michael stepped in or not.