The problem with doors in New York in particular is that not all generations of rolling stock have doors in the same location. Unlike MTRC, where one line has its own rolling stock and there is no interoperation between different lines, many services, each with different door positions, can utilize the same platforms. And basically the only way to have a door configuration compatible with all rolling stock is to not install any doors at all.
You could replace the rolling stock, but all of it is in good operating condition, and the last of the oldest cars with non-compatible doors would be retired in the 2030s.
Air conditioning is also a huge challenge; traditionally, New York's subway has been cooled using the piston effect of trains, with frequent grates open to the surface to draw in air. With air conditioning those large, frequent openings are now a curse.
You could replace the rolling stock, but all of it is in good operating condition, and the last of the oldest cars with non-compatible doors would be retired in the 2030s.
Air conditioning is also a huge challenge; traditionally, New York's subway has been cooled using the piston effect of trains, with frequent grates open to the surface to draw in air. With air conditioning those large, frequent openings are now a curse.