I think ya'll are off on this. As Vito said, he didn't want all this for Michael. He wanted him as a respectable politician to hold the strings and part of his plan to accomplish that was to have Tom as Consigliere. Tom understood that war was only ever a last choice. Vito knew that about Tom and figured that Tom could pave Mike's way into the legitimate world.

That both Sonny and Mike did not think Tom a wartime Consigliere was their collective recognition of the truth. Tom was not prepared for the endemic ferocity of life in the Mafia. In that way he was not a Sicilian. He also was not a Sicilian in that he was just not very cunning. Vito's calling the Dons together was, in part, a recognition of Tom's insufficiencies because he realized that the Corleones could not win the war and, as the novel says, it would take the Don's strategic genius to retrieve the family's position.