They haven't extended the T since I think the 80s (Not counting those silver line buses). The green line extension is coming anytime now....
Massachusetts has a hard time getting stuff done, but it does seem to happen eventually (convention center, new garden, big dig).
Part of the problem is the lack of state (or county which barely exists here) control to push stuff through. The red line doesn't go to Arlington because they didn't want it. The garage at Ailewife station doesn't have enough spaces and the T doesn't have the money to expand parking to meet demand.
You are right, you can sit here all day and come up with improvements.