Toronto did the same thing in 2015. The agency now estimates that they lose $80 million a year (~13% of their revenue) due to fare evasion. This is purely anecdotal, but there definitely appears to be higher crime rates on the subway system since they made this change. However, the chair of the system said that making sure everyone has paid is the first step to addressing the “lawlessness” there.

While it would be nice, ignoring the issues with rolling stock, doing such a think would cost several billion dollars (based on case studies of other systems which have considered adding them). As of now, if the MBTA had that money, it would be better spent elsewhere making sure we actually have a functioning system where trains don't burn to the ground.

The Toronto Ontario Line is meant to have them, however knowing Toronto, it will probably be 2050 before it's finished.