What flew for acceptable rapid transit in 1959 does not fly in 2019. What works in Norfolk with a few hundred passengers a day does not work in Somerville with 5,000 passengers per day at stations. This is not a surface streetcar line, and this is not Newton with nicely sized residential lots and light walk-up traffic.
This is Somerville, with triple-decker density surrounding almost all the stations. Every single station is going to get passenger loads 2 to 4 times the typical D Branch station, much closer to actual heavy rail passenger counts. So you need to build your stations to accommodate those loads. That means grade separation, no bullshit with track crossings. They are not safe for passenger loads that high, they are less safe for people in wheelchairs, and having to wait for floods of people to cross the track will absolutely murder your schedule reliability.
The D Branch is not a good comparison whatsoever. Only five stations actually force track crossings (Brookline Hills, Beaconsfield, Newton Highlands, Waban, Woodland - all less than 2000 riders/day), and all to the inbound platform. The others have an entrance/exit to both platforms. That means that when you have the largest crowds at once, many of them can directly exit the platform. That simply would not be true at Ball Square and Lowell Street, plus Union Square (island platform needed as a terminus without room for tail tracks).
The vehicle is the technology, not the service. The service that Somerville demands and requires is rapid transit, regardless of whether that comes as Green Line, Orange Line, or Silver Line. That means a service where passengers are separated from the paths of the trains at all times, with station designs suited to handle massive crowds. The south end of the Orange Line is a much better comparison for what level of demand we're looking at. Those concrete bunkers wouldn't be any cheaper than the stations as they are currently designed, especially if the MBTA got sued again for accessibility requirements. Ramps are not suitable for actual mass access, and dual elevators are the name of the game. POP saves absolutely nothing for these stations except the actual faregates - you can't actually skimp out on much else without making the stations less usable, less safe, or less likely to pass code. By the time you go through literally years of redesign - in the process completely fucking over the community through delays and through stations that don't serve them as well as these do - you're not going to save any money no matter how much you fantasize about third-world asphalt platforms.