Agreed, boroughs with high transit ridership are going to be the big hubs, but very few of the people going through those hubs live in the immediate vicinity of the hub, and the people who do live in that vicinity may not have a lot of direct contact with those commuters. The whole problem with transmission through subway transit is that the people using that transit scatter to all and every corner of the region and beyond. They don't stay concentrated in the immediate vicinity of the transit system.

I use an inner city transit system every day on my usual commute, but the transit system hub I access it from is about 15 miles away from where I live. I use a bus and then a regional train network to get to it, then go into the underground.