500 feet isn't even that tall for a city like Boston, especially in a neighborhood like Downtown Crossing. What's the issue with building (or at least zoning for) 500' towers in DTX when the Millennium Tower is 684'?
There aren't any current plans for it. A Red Line extension to Mattapan was planned back in the 60s, but there was a considerable amount of opposition.
Business foot traffic is up within the congestion pricing zone
https://www.crainsnewyork.com/transportation/congestion-pricing-zone-business-foot-traffic