Michael said, "After all, who's a better consigliere than my father?" But he seemingly put Tom back in the consigliere role after Vito died because he needed him. As you saw, by the beginning of II, Tom was out in the cold again--until the Tahoe shooting, when he became "the Don."
Classic Michael: use, use, and use some more.
"The only wealth in this world is children. More than all the money and power on Earth." --Michael Corleone
