The sensible way to get to LGA is to extend the N/W line. You get ~two more stations in the local neighborhood, plus the station at the airport itself, and it's

The sensible way to get to LGA is to extend the N/W line. You get ~two more stations in the local neighborhood, plus the station at the airport itself, and it's a direct stop on the local subway anyways, no interchanges required (except as necessary to get to the N/W). By contrast, the LGA Airtrain idea is to go the wrong way. This extra detour means that existing transit (i.e. buses) is likely to still be a faster way to get to the airport. It also requires a separate transfer, with a separate, steep (IIRC) fare for what it's doing.