Suburbs are dense enough to support transit as proven in a small number of cities around the world (mostly not English speaking). However suburbs can only support great transit, since anything less and driving is enough better as to be worth it.

NYC has a real cost problem. Digging a tunnel costs a lot - not anywhere near what it costs in NYC. You can also build bridges over the top for a lot less than digging a tunnel. Modern subways should be 100% automated saving the cost of a driver. (I keep hoping we see self driving buses since drivers are the large share of the costs)