PB, as you know, a lot of people here agree with you. Hard to argue that when he said, "I'm with you now, Pop," he didn't just mean that he was physically by his side.

You could also argue that his decision to whack Sol and Mac was tactical--to protect his father--not necessarily a commitment to becoming Don, especially since Sonny was alive and was running the family.

SPOILER:

In the novel, Michael is unconscious for several days after the bombing that killed Apollonia. When he comes to, he tells Don Tomassino, "Tell my father to get me home. Tell my father I wish to become his son."