I think early nineties is when the Feds started to break these guys grip and it's been slowly declining ever since
Yes, 1992 was the year that really got the ball rolling.
It all started with the Teamsters. The info the Sammy gave the Feds about Local 813 beginning in late 1991, had Bernie Adelstein out of office by the fall of 1992. After more than fifty years running the local, Gravano's testimony unseated him in less than a year. And the information which led to Adelstein's ouster is what led to Jimmy Brown finally getting pinched on a murder that the Feds never would have convicted him of otherwise.
Getting Jimmy Brown out of carting and out of the union (he was Adelstein's Rabbi since forever) was a huge blow to the New York Teamsters. It had a trickle effect. Local 813, Local 456, Local 282, and on and on. Eventually the Federal mandates would go nationwide. Today the mob getting next to a big time Teamster local is next to impossible because of those mandates. And if it does happen in a rare case, it won't last long. When you can ban a guy from the union for life because he "brought reproach upon himself" by virtue of having had dinner with a guy once twenty years ago, you know the deck is stacked.
That's what gave the Feds the gumption to eventually go after the other unions. The conventional wisdom being that if you can unseat the mob from the Teamsters, which at one time was unthinkable, you could eventually chase them from the other unions as well. But they've proven themselves to be harder to remove from the waterfront and the various laborer's locals (carpenters, steamfitters, etc.). In my opinion that's because the wiseguys weren't as greedy with those locals as they were with the Teamsters. They treated the pension fund like it was their own private lending tree, and they paid the price for it.
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