Well-posed scenario, Goombah.

I interpreted the passage in the novel to mean that Sonny was waiting for Michael to drop his Ivy League persona and help Sonny wipe out the "f***s who are trying to kill our father." I didn't interpret it to mean that Sonny wanted Michael in the family business (not what Vito said he wanted for Michael), but it's not impossible to imagine Sonny needing Michael's help down the road, or if Vito had died. BUT:

Vito wanted Michael to become "Senator Corleone, Governor Corleone..." But I think he wanted Michael in high office so he could help with the legitimization of the family enterprise. As Senator or Governor, Michael would work, with Vito's help, to legalize gambling in New York or nationwide--the better to channel the Corleone gaming empire into legal casinos (as Michael later did in Nevada). Michael could also use Vito's labor support not only to help him get ahead, but to gain political allies for the cause by channeling labor money and votes to other politicians who could be converted to legalize gambling).

Sonny would head the muscle side of the business during this transition. That would be an ever-diminishing role if Vito's scenario played out. I can readily envision Sonny resenting his little brother, leading to conflict--and violence. It'd make a swell story for another movie or novel.