unsustainable is an absolute, not a relative. A road network doesn't have to be unsustainable relative to anything in order for it to be full to capacity at and for there to be no room for any more roads, which is the situation New York finds itself in. It is physically impossible for NYC to have sustained substantial growth to their private vehicle infrastructure, hence "unsustainable". If they want to grow their transportation network, they need to do it via other modes.

And i reject your assertion that New York has the best possible conditions for transportation infrastructure. A lack of space, a huge demand, and a corrupt mixture of local government and unions don't make for an idea development environment.

Maintaining existing roads is certainly not beyond the economic capacity of the United States. It just costs more than you're willing to pay. That's valid, but the alternatives don't appear to be cheaper. New York also finds maintaining its subways in working order, something that's certainly physically possible, to cost more than it's willing to pay.

High density and high transportation demand are prerequisites for having a public transit system at all. And what dense US city's government isn't corrupt or incompetent?