I wonder what % of traffic is to/from JFK. The subway decently connects much of the city to the JFK air train, but it's a fairly inconvenient journey. Toronto's UP express has made travel to YYZ significantly easier, but I doubt it's possible to construct something similar in NYC.
I love aviation, but I also don't see air travel as being a scalable/affordable solution to this problem. Then again, it's only meant to alleviate traffic burden for a certain segment of the population.
if your air taxi is pilotless and electric, why it can't be scalable.
the problem with train it stops ... on every train stop. New york specifically, there are several networks(new jersey, mta, there are lines that are 100+ years old.
In general if you have an affordable enough option you'd never walk into subway, with your several luggages, to travel longer. Train is a decent plan b.