That's an interesting question, Olivant. I'd argue that the Corleone fortunes declined because Vito was shot and incapacitated--and that was because he underestimated Solozzo. Also, apperently he didn't include Tom in his plan to use Luca to try to get close to the Tattaglias--a patently ridiculous idea that Tom might well have advised against.

On the other hand, Tom truly wasn't a wartime consigliere. I don't think it's just an empty phrase. As he said (in the novel, re. the Sonny ambush): "Old Genco would have smelled a rat." Not only that, Tom probably was overcautious. For example, he told Sonny that, as long as McCluskey was guarding Sol, "he is invulnerable." But if Michael hadn't prevailed, I'm certain Sol would have gotten another (probably successful) shot at Vito. As we've discussed before, his training as a lawyer prepared him for conciliation, whereas Genco's background as a Sicilian prepared him for cunning, and to anticipate cunning.