What would happen if Vito Corleone decided to take Tom and Sonny's advice and agree to Sollozo's deal with Narcotics

And within forty years of when the movie was set, goombahs using their own product helped weaken the Mafia.

In the short term all the families involved would have made a fortune. And they would not have lost the manpower that was lost during the war. And Don Corleone would have remained at the top of his family for a longer stretch of time since he would never have been shot.

Long term, Barzani would silently be gaining large amounts of money from the drug trade without the Corleone's even knowing he was involved, since there was no meeting between the five families. Just as Tom had predicted, they used that to buy more police and political protection. Once they had all their pieces in place, they would start going after the Corleone's territory, and eventually take them out.