We can see a theme with the city trying to have subways outcompete elevated lines so that they could be knocked down. The South 4th Street truck line would’ve probably competed with the BMT’s Broadway Line in Brooklyn (the J/M/Z). The M would’ve gone down Myrtle Avenue, with no connection to the South 4th Street Line, and the J/Z would’ve either terminted at Myrtle Avenue or merged onto this truck line.

In addition, at the other end of the J/Z lines, the Hillside Avenue Line would’ve competed with it for passengers by being so close to it, so there probably would’ve been another transfer station built at that end as well (so the J/Z could potentially be reduced to a second Crosstown Line).

That Long Island City Line is a BMT line, not an IRT line (it is yellow and branches off of the 60 Street Tunnel, not the Queensboro Bridge.

As far as all of those extensions, the advantage is that the MTA would be able to save money by running fewer buses in Eastern Queens (a train has higher capacity and costs less per rider than a bus). You see how all of the routes in Eastern Queens, as well as Whitestone, tend to have a lot of riders, that could be taking 1 train instead of 10 buses.

If only there was a version of Robert Moses that was pro-transit instead of pro-car. We would really have a good transit system then.