I may have had to reach back, but I sure did have a perfect counterexample, didn't I?
As to the 'heroics' you've seen in NYC, I'm sure you have seen plenty. With more people there, odds are good you would see more 'heroics.' Heck, I'll even bet you do see more than I do in my lifetime.
But that doesn't mean that the town is full of heroes.
And btw, war zones don't make heroes. Escapable yet willing self-sacrifice makes heroes. Someone who shoots an enemy soldier with a gun pointing back at them is not necessarily a hero. Even someone who takes out a whole enemy regiment isn't necessarily a hero. But someone who willingly exposes himself to mortal danger to save others, when he could easily escape that danger himself--that's a hero.
Kitty G became a symbol for an uncaring city, etc. etc. etc. The city was viewed in decline, as compared to the suburbs of Long Island and Westchester. Less well known is that in those days fights between husbands and wives, girlfriends/boyfriends were frequent and loud. Nobody ever reported them. So that was a factor. I could find a hundred similar incidents out in the burbs or rural communities of brutality that people turned their back on.