The Mob has been deeply damaged by relentless federal prosecution via double-digit drug and RICO sentences. Also, greater opportunities in the legitimate world, coupled with greater assimilation of Italian-Americans of Southern Italian descent, have diminished the pool of top-notch operators--compare Gotti pere et fils with Carlo Gambino.
Selwyn Raab, in his outstanding Mob book, "The Five Families," acknowledges, on the one hand, that federal prosecution and assimilation have weakened the Mob. But Raab also notes that, since 9/11, many of the resources that had been directed at busting organized crime have been redirected toward anti-terrorism. He sees this as a vacuum in which the Mob can renew itself.
My view: As long as people want to do things that are prohibited by law--gamble at places other than racetracks or casinos; buy and use drugs; borrow money they're not entitled to; receive stolen goods; patronize prostitutes, etc.--there'll always be criminals and criminal organizations to satisfy their needs. But I doubt that the Mafia will ever be as big or powerful as it once was.