The people arguing that removing part of the mall for offices and housing next to transit will increase traffic set my brain on fire... how sway? You aint got the answers man...
The thing is that these people aren't
wrong, they just can't see (or don't care about) the bigger picture.
Removing part of the mall for offices and housing
will probably increase traffic in this specific area of Cambridge. So if your scope of analysis is this specific area of Cambridge, it makes perfect sense to oppose this project on traffic grounds.
Of course the project will
also likely decrease regional traffic relative to an alternative in which the people who would live and work in these buildings live and work somewhere else in Greater Boston instead. But that's more subtle and doesn't fit into your calculus if all you care about is this specific area of Cambridge.
This is why NIMBYism is so hard to overcome. If you care about your "back yard" more than anything else, it makes perfect rational sense to oppose projects that benefit the greater good but come with changes to your back yard that you don't like. There's no way to convince people with this view that they're wrong, because really, they're not wrong. They just have their sights set too much on the micro when broader societal improvement needs to happen on the macro.