Don Corleone and Don Carlo were right. Drugs=easy, fast money. Easy, fast money=guys became greedier=more guys dealing=more guys get caught. Drugs=long, long sentences=more guys caught=more guys flip=downfall of cosa nostra.
...yet it seems there's never been a time in it's illustrious history when somebody somewhere was not trading in
babania...
Gotti also mused in the '70s that "all of the mutts running around"(half-breeds, half-Italian and half-whatever else) was bad for cosa nostra. They couldn't be trusted he said. -Sinatra Club.
Much as it cringes me to say, this fits. And led to...
...a general disregard for all of their organizational rules led to a crumbling from the top in Cosa Nostra. Too many/any passes given, guys start developing a lack of respect, start taking shortcuts, all lead to an overall lack of honor and respect. Then it doesn't matter what they do, they won't answer to anyone.
...so to sum up...
...generational changes in the mob, increased penalties from the law) had more to do with the downfall of the mob than drugs itself.
...but my final answer awaits two guys yet to pipe in on the subject...
