Michael would be Don anyway. He tested himself and discovered so many interesting things ! And he tasted that life, and his Sicilian roots - and liked them. Remember this thinking of his, in Sicily:"He was, after all was said, a murderer, a mafioso who had "made his bones"." You can'd expect a Mafioso to make his bones for nothing, even if he is half as formidable person as Mike is.
Even if there would be no Solozzo, something would help him. Vito would make him his successor in some more elaborate way. Or he would kill someone else. He was destined to find out what he really was (how many of us can't imagine that-thank God )and you know, man has one destiny .
And why couldn't he step over Sonny? Mike was not a man to act less ruthlessly because of his respect to anyone. "It's the way pop wanted it. Sonny, old man, it's not personal, strictly business, you must know by now."