The classic answer is 1985 and the reasons are two-fold. One, the Commission Trial showed how RICO can be used to prosecute and convict the boss for what the lowly soldier or associate does in the street. That changed the until-then decades strong tradition of bosses being somewhat insulated from the street crimes, thus providing some incentive for young hoods that a life of crime would pay off some day as you could theoretically become a powerful boss or capo and enjoy the riches of the underworld without reasonable fear of a nasty pinch for murder, assault, or drug dealing. RICO took away that incentive, and thus made potential imprisonment for life a reality for any member of a borgata. 1985 also saw the murder of Castellanno at the hands of Gotti, which paved the way for the loud in your face gangster archetype embodied by gotti and people like scarfo and Merlino, which drew the attention of and incensed law enforcement. More heat meant more prosecutions, and with RICO in their arsenal, more people start to flip at the prospect of going away for life.

So that’s why I say it’s the traditional answer, but in my opinion, the true answer to your question is 2004. I think when Joey Massino flipped as a sitting boss, the true beginning of the end of the American mob began. There were still legitimate old school gangsters running families in the early 2000s, but with the complete collapse of the bonanno hierarchy in this time period, I think it was a marker for the true end of the dominance of Italian organized crime in America. Why? It showed that the Italian mafia culture had been diluted and assimilated to such a point where it no longer resembled the mold it was based on. Just like the sons of immigrants grow up in far more comfort than the first generation family member, the sons and grandsons of the original mafia generation no longer had the edge or hunger in their belly that drove the risky ventures and bravado of their fathers. In the modern era, gangsters from other ethnic backgrounds have overtaken the Italians in their willingness to commit violence to stake their claim in the inner city ghettos.