Cities are likely to be much safer today than they were when you were growing up. It's the adults' attitudes to seeing kids alone and unsupervised that changed.
Uhhh no. When I was a kid there was viturally zero chance of running across used needles at the park, nor being harassed by homeless people on public transit. Look I want to make cities work as much as the next guy, but at some point I’m going to vote with my feet and signal that things are not acceptable.
Any homeless person who's not completely bonkers knows that harassing a lone child on public transit is just about the worst possible way of attracting the attention of law enforcement, since other adults might ignore harassment of other adults who seem to be dealing with the situation but will step in to protect a child. In all of my life living in large cities, I have never seen something like this happen, nor came across any news of something like that.
This seems to be in the same category as not letting a child walk alone to school, in a Western country, because of fear of Islamic terrorists -- possible, but so unlikely it is statistically improbable.