Every single time this comes up, people neglect to mention that
-The LM helps keep tons of freight out of the beds of trucks, helping the environment and traffic by serving most of West Queens's industries
-The LM keeps the LIRR Main speedy because it prevents LIRR and NYA traffic from conflicting within the city
-The majority of areas served by the LM are transit deserts, sure, but also are little-populated industrial backwaters that don't need subway service as pressingly as, say, Southeast Brooklyn or Southern Queens.
LM passenger revival would also either need time separation (which conflicts with Waste Management garbage operations) or FRA-Approved stock (which would be an investment, especially if it was stock the LIRR could not maintain.) This will remain a pipe dream for the forseeable future.