“ let’s say we build 30 miles of subway, about what Manhattan has in that area.”

Public transportation that connects to nothing is not useful. The NY metro area has a massive public transit set of systems that extend out for a hundred miles in every direction. That’s why you can support such density.

edit: It's 665 miles of MTA track, 14 miles for PATH, 700 miles of LIRR train tracks, and 385 for Metro North tracks. Plus Amtrak and the regional buses (the new depot alone for those will cost $10 billion).