Some Dons, like Carlo Gambino, Frank Costello and Tony Accardo, were as effective as Vito Corleone. Most others weren't. Many made fatal mistakes. Even Gambino erred when he appointed Paul Castellano as his successor, instead of Neil Dellacroce.
RICO and drugs hastened the demise of the Mafia. But larger trends were at work, namely the complete assimilation of Italian-Americans, which made a Mafia career irrelevant to the best and brightest.