That is an interesting idea....but the idea was that Vito wanted Michael to be more legit...if that was not possible (as it had worked out) then Sonny would have been the wartime consligliere. Remember, Sonny was hot tempered, but could be controlled. As you read the book, and view the movie, remember that as he is needled by an action (like sitting at the dinner table discussing business, and Connie says, 'Poppa never discussed business' he remembers, and agrees) he focuses back on what was required of him. As he aged, he would have mellowed, and became a true underboss to Michael.