There is no restriction of through traffic. Effectively pedestrians are still confined to tiny and overcrowded sidewalks.
By comparison, here’s what a pedestrian street looks like in the non-US city I grew up in: https://sana.ae/wp-content/uploads/2020/04/Corso-Italia-Stre...
Keep in mind that cars are still allowed for emergency services and (night time) deliveries. But the difference is night and day.
This is exactly what “the US can't imagine itself without cars” means to me.
If you think population density is an excuse for public transport infrastructure not coping or need for more people owning cars I suggest taking a long hard look at e.g. Japan to have that hypothesis reality-checked.
I'm btw. not saying you did, just reading between the lines.
As I wrote in an earlier reply to parent, NYC hasn't managed to even build ring Metro lines around its city center – since a century!
And that is for one reason and one reason only: not nearly enough (political) pressure from the public to improve public transport infrastructure.
And that in term gets us to the root cause again: the US can't imagine itself without cars.
This is not a critique. It's just an observation that is very plain to see if you grew up in Europe (and possibly many other places, too).
When/if that changes, ever, the above things will just happen naturally.
Many metro systems in other big cities with comparable topological constraints have metro lines that are orthogonal to those going out in roughly a star pattern from the center.
But NYC almost only has the latter.
Underwater sections are not an issue, really. There are many cities that have metro lines going under bodies of water.
And even the length or depth required is not an issue if you want to build it.
That's why you can go from England to France, by train, in roughly half an hour, under the English Channel.
They just don't look like streets anymore, as they are turned into plazzas or parks.
EG: E25st at Lex, Baruch College is truned into a plazza/walkaway. No cars. 8th/st Saint Marks, by A Ave, is off cars, (It is part of the Tompkins park). Irvin Avenue is part of a park (gets interrupted by Grammercy Park)
etc...
There are plenty of places like that, but over time they turn into plazzas or parks, and you think they were not streets at some point.
Ps https://maps.app.goo.gl/Df6U3DkPpUxirG5B9
https://maps.app.goo.gl/ZivSNhiEnn2Q4pjr6
You can see throw time that it used to be a street at some point.
Anyways, there are plenty of examples like that. Just stop exaggerating.