I think we have a few years now of pretty solid data on how much delay red line riders are willing to suffer. The imposed delay has been in the tens of minutes, and some folks definitely switched to the commuter rail, but it's not as if the braintree branch saw utter collapse. My anecdotal experience is that I found I actually would get to my office faster biking to QC and taking the commuter rail as opposed to taking the red line from QA....and I still only ever did it when the red line was actually shut down. There's other factors at play in people's commute modal choices, such as the experience of getting to the station which, for me, was horrible for QC.
Additionally, I think most red line riders intuit the unusually long gap between NQ and JFK. I think most would find an additional stop (be it Savin Hill, Neponset infill, or both) to be logical.
