If we take III into the equation, ALL of the problems that happen to him in that film stem from his refusal to let his fellow Mafia bosses share in Immobiliare, in order to be legitimate. Had he cut them in and worked as a gangster, no assassination attempt, no war, no dead daughter.
That's debatable at best. If he had brought other (rival) mobsters like Don Altobello or Joey Zasa in on the Immobiliare deal they would have become more powerful and therefore a much bigger threat. He would also instantly ruined his change of ever becoming legitimate. I think Michael made the right choice to keep them out of it.
