> Airports are a particularly visible omission, for a certain class of people, but they aren't in the top 5 or so most valuable possible extensions to the subway system.
Speaking of class, some transit systems suffer more than others from lacking a healthy mix of ridership affluence (MTA isn't one of them). Descending into a spiral of "only for the poor" is one of the failure modes of transit systems. If an airport line "injecting" its share of non-destitute riders is what keeps the while system from falling off that cliff it would be worth far more than just their tickets or the directly prevented road traffic.
High status destinations have indirect benefits for the whole system.
Airports aren't in the top five most beneficial routes for middle- and upper-class residents, either. There are all kinds of outer borough routes that would be of far greater benefit to the residents there.
This is sort of like the case of the couple who buys more house than they need because the in-laws visit once a year and they insist on having a room for them. It's silly to optimize for rare events. When you have to go to the airport, then it's particularly noticeable that there isn't a route to the airport. But going to the airport isn't on your everyday agenda. There are far more beneficial routes.
Speaking of class, some transit systems suffer more than others from lacking a healthy mix of ridership affluence (MTA isn't one of them). Descending into a spiral of "only for the poor" is one of the failure modes of transit systems. If an airport line "injecting" its share of non-destitute riders is what keeps the while system from falling off that cliff it would be worth far more than just their tickets or the directly prevented road traffic.
High status destinations have indirect benefits for the whole system.