The most important criminological trend of our time is completely ignored

[Benton D Struckcheon]

6. Yeah, it's probably self-reinforcing,

where the drop in the population of males that age partially explains it, better policing partially explains it, fewer "mentors" now partially explains it, and the latter two have a persistent effect even when the population of males the right age rises again, because the police still have things under control due to having learned a few things over the past 20 years.
But everyone is actually well aware our crime rate has collapsed. This is especially true in NYC, where pre and post Giuliani NYC is like night and day. Better policing, which actually started under his predecessor, Dinkins, was aggressively rolled out to the entire city after conspicuous success on 42nd Street, and the city has been a lot safer since. A large part of the stats you cite is directly due to NYC having become so safe over the past two decades.
It's not a good argument against gun control though. NYC as I have noted is where things have really improved tremendously, and NYC has very strict gun laws. NJ is reasonably strict as well; we have universal background checks here (I know because I once did a friend a favor by signing off on his competence to own a gun; you need two people as I recall to testify to your being sane on the form). So is NY state, so NYC benefits from its immediate surroundings being decent on gun control, which makes its own laws more effective.