In some ways, yes. Many of us here believe it came even earlier: when, in the hospital, he said, "I'm with you now, Pop," he could have meant that he was, finally, with the family, not just at his father's side.
But I believe that he could have chosen to go his own way after Sicily. He could have said to Vito: "Pop, I was wrong to hold you and the family at arms-length, but I made up for it. I killed the two guys who were conspiring to kill you, and saved your life. I paid a heavy price: I had to ditch my beloved Kay and hide in Sicily. I lost my beloved Apollonia to a bomb intended for me because I'm your son. We're quits. Sonny dead? I grieve as do you. Fredo not up to the Donship? Sorry, Pop, but that's not my problem. Besides, you always said you wanted me to be 'Senator Corleone, Governor Corleone.' Well, Pop, I can't get there if I'm 'Don Corleone.'"
He may have been able to leave the Family after Siciliy, but where would he go?
Aspirations of holding higher office were basically killed along with Sol and Mac. And the other Families would never view him as a civilian again.
The closing of legitimate avenues to Michael was made quite clear by Vito's reaction when he's told that Michael killed Sollozzo and McCluskey.
