>Where it exists you can see that there is tremendous demand for it.
Everybody loves public transit until they get panhandled for the jillionth time, or they witness (or experience) violence, or some other anti-social behavior sours the whole thing.
I spent some time in NYC during the Giuliani years, after the city did a lot of work cleaning it all up: stopping turnstile jumpers, removing graffiti, more police, etc. It was great. You'd get the occasional guy that jumps on, makes a speech about how he's raising money for something or other, and walks around trying to sell chocolate bars. And there was the occasional dangerous person, insisting on getting up in your face.
So long as this sort of behavior remains at a very low level, something like maybe once every couple of weeks, that's probably okay. But public transit loses all appeal if it happens often. If it rises to the level of violence, everybody starts thinking about the suburbs.
Public transit requires a certain level of unspoken agreement. "We will all behave in this manner." If this unspoken agreement is broken often enough, then it must be enforced. If it is not, and other options present themselves, people will choose the other options.
This happened en masse many decades ago in America. Those that could decamped for other places where their social expectations were met.
I'm a big supporter of urbanism. I loathe the time I spend in my car, and I don't even have that far of a commute, but I have zero other options if I want to live where crime is low and the schools aren't dysfunctional. Until this is addressed, there is no argument about commuter density or efficiency of movement or anything else the proponents of public transit like to talk about that will make a lick of difference.
The worst argument anybody can make is "but that's just life in the big city!" If so, then I'm not going to live and raise my family in the big city. Airy-fairy principles of efficiency or an arguable notion of convenience will not take precedence over safety and quality.