I have been to NYC a few times and I always had the feeling that if I had a heart attack and fell to the sidewalk, people would just step over me and keep going. I felt like I was invisible. I hope I am wrong.
Houston has become almost that big and extremely crowded in places and people will still smile, say hello and hold the door open for you. I try to do that just to keep the tradition alive.
Contrast that with having a heart attack in say, Douglas, Alabama, in which you are an hour from Huntsville and an hour and a half from Birmingham. The people helping you might be more polite but they will not likely be able to save you. With cell coverage spotty, you might not even be able to get an ambulance in time.
I had quite the opposite experience to yours. I found them extremely helpful. Just stop for a few seconds and look puzzled and somebody will ask if everything is ok or if you need help. NYC changed dramatically after 911 and is much more friendly than other world-class cities. If you fall down and don’t get up you may be mistaken for a drunk or panhandler, but somebody will eventually check you. If you clutch your chest or trip, as you are falling you will likely be caught by alert stranger; I trip over my feet quite often on slick pavement and can attest to this.