I mean, in a perfect world they would be EMUs....
If they bought a bunch of EMUs, it would free up all the diesel locomotives and cars currently used for the Providence line, which could then be used to replace some of the single level cars on the other lines. Which would be a better outcome since the Providence Line would then enjoy faster service.
They should be electrifying Fairmount, Stoughton, Needham, and maybe Franklin, buy EMUs for those services, and then use the locos and bilevels freed up by that to retire the single-levels and GP40MCs, instead of buying new coaches and diesel locomotives.