Sonny, we might be able to account for that long apprenticeship by observing (as Tom told Fredo in the scene with Moe Green) that the Don was semiretired and that Michael was in charge--that he was the day-to-day Don of the Corleone Family. And, he needed his father's prestige and his counsel.

But, realistically, Vito admitted that his agreement to protect drug trafficking at the Commission meeting was a sign of weakness. And, the novel tells us that the other Dons, and even Tess and Clem, thought Michael "lacked force." Michale himself was feigning weakness to buy time. Over a period of seven or eight years of that, the other families would have taken advantage of that passivity to reduce the Corleone holdings to insignificance. As Tess said in the "fish tank" scene: "Pretty soon I won't have a place to hang my hat."