Tom I agree with half of what you said. I have serious doubts that anyone will be able to take the place of the original actors and make it believable. However, Vito and Michael are dead, so is the story. The Trilogy has always been about the father and the son, and seeing how they are both 6 feet under, there is no more story to tell. If there are no actors to fill the roles, and no continuing story to tell, the logical thing would be to not make a Part 4.