All that you said is true, dt, but there was more: Vito held an ace in his hand--police/political protection for the drugs business. If the Dons hadn't agreed to allow Michael to return, and not to move against Michael, the protecton would have disappeared, even if Vito didn't have the muscle to carry out his threat.
So are you saying if Michael had been hit by a bolt of lighning, that suddenly the police and judges might stop protecting "some of the people in this room?"
