The MBTA would L O V E to bustitute this line. It would be such a loss. But extending the heavy rail Red Line would require an overpass at Central Ave and Capen St which is 1) expensive and 2) going to bring the ire of Milton NIMBYs (especially along Eliot St). You'd basically have to have Milton and Mattapan come out and fight FOR Red Line to keep seeing trains down there and I don't think that's going to happen (Mattapan yes, Milton no).
An RL extension would only have 2 stations, Lower Mills and Mattapan and I'm not sure Lower Mills can really justify the cost. Mattapan would be a more obvious choice if it wasn't for the new Blue Hills Ave station going in. You know the MBTA is going to say that's enough and call it a day.
IF the MBTA is going to keep running this as a trolley then you need to see the state rep from Milton step in and find funding to buy some new trolleys for the line. It would be nice to have modern low floor cars to make the line ADA compliant. The state rep, William Driscoll is on the
Transportation Committee so maybe that's a help.