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.