Let's say Sonny had managed to kill Bruno Tattaglia, whom he believed to be behind Sollozzo. With Vito still alove but weakened, such a move would have probably forced the remnants of the Tattaglia family and all the other families into an alliance with Barzini, and perhaps other families from out of New York, including the Molinaris on the coast against the Corleones. To that end Tom was right in his advice to Sonny...that with the stories about McCluskey circulating, things were starting to loosen up and money was starting to come back in. At that point, the family was considerably weaker han it had been, and Tom was absolutely right to tke a wait and see approach.

All the sibling rivalry stuff that we have covered in other threads considered, Sonny refused to take Tom's advice, assuming he was not a wartime consigliere like Genco was, and made the incredibly bad decision that "business will have to sufer."
This is 180 degrees opposite to what Mike and Vito eventually did. They went out of their way to lay low. They knew the family did not have the kind of muscle it needed to win a war, and they knew they had to keep things greased lest the other families turn on them (which they were already doing, i.e. chiseling away at Tessio and Clemenza's territories). Michael secretly built up the Neri regime, something that did not escape Tom's eye, and when the time was right he mde his move. His brilliance was in the timing. He waited until he had the muscle, until after the Baptism and his being named Godfather to Connie's child before he paid his "visit" to the heads of the families and to Carlo.

All of this is pretty much in line with what Hagen would have recommended IMHO, and in fact Hagen played an integral rle in the planning and execution of the hit on Tessio, which he handled brilliantly, as well as the hit on Carlo. He was also an instrumental layer in the hits at the end of GFII. So what I am saying here is that while Sonny and Michael were always quick to dismiss Tom as not a wartime consigliere, perhaps he was not as bad as all that after all.