More than half of the riverbank subways potential service area would be water, or a park. The rest is by and large low density brownstones, occupied by people who are probably walking or being driven to work. The only stop that would make sense is at the Hatch shell, since you get some Beacon Hill traffic as well, and one at Mass Ave since it's built up more over there.
This works well for the Blue Line, since it would serve as an express bypass. It wouldn't work with the green, which needs local stops and bread and butter is serving the High Spine.
Extending the Huntington Subway to Brookline Village in the west and the disused tunnels out of Boylston in the east makes infinitely more sense to do something about the Green Lines capacity woes. It still serves the high spine, and takes advantage of the *relatively* easy to dig in soil next to the Pike, which is presumably free of undocumented utilities, boats, and buildings. You also have the opportunity to tie it in to the Transitway.
