This is related to the other thread on Vito's drug fears. The argument Tom Hagen made to Vito was that if the Corleones didn't get into the drug business, the other families would, and in time, maybe ten years, they would have enough money to buy political cover for themselves and then they would come after the weakened Corleones. Assuming this was correct why didn't Barzini just finance the drug trade, and buy off politicians, allowing Sol and Tatt to be the fall guys, and then move in on a weakened Corleone family?