^ In general, I agree, and I certainly don't think that BRT along either the old BRB&L RoW or the current commuter rail RoW is a good idea.
However, in this case, I imagine that "BRT" would consist mainly of bus lanes on Route 107, Route 1A (North Shore Rd), North/South Common Street, and Market St (
some of which is already being built), plus potentially some other "on-the-road" improvements like level boarding or pre-payment. Being an order of magnitude or two cheaper to build than HRT, I think this would be less likely to fall victim to the sunk cost fallacy.