Decline in gun violence in the United States
washingtonpost.com
washingtonpost.com
"Gun homicides didn’t decline 49 percent over the last 12 years -- rather, they declined precipitously over six years in the 1990s, then have remained basically stagnant for the past decade or so.
Why is this important? Because the decline, to a large degree, has to do with a specific reason -- the end of the crack epidemic."
Unless the graphs for mitigating factors show a similar (or mirror) shape, then it is unlikely the reasons cited are valid.
Not that any but a 1st half of the 20th Century progressive would be happy with the possible mechanism you mention.