I've never heard someone give the subway as a reason for leaving NYC. They leave because they're priced out of what they consider acceptable housing, the weather, or issues with the city itself. But never the subway.
Yeah of course, but there's also the people that got sick of the subway and bought a car, right? Hopefully they could buy a bicycle instead but with theft being what it is and how dangerous the streets are there's even less people willing to do that...