And the population of Greater Boston didn't decline, either, even with the white flight from Boston. It, in fact, has grown only grown during that period (with perhaps a slight decline for the first time due to the pandemic). It is the area serviced by the MBTA, which is a state agency and services significantly more than just the City of Boston itself. Furthermore, the MBTA took over from the MTA in 1964, and certainly hasn't been planning the deficit game since then, and was quite expansionist for most of the period up until 2000. Previous to the switch to 'forward funding' in 2000, the MBTA couldn't take on debt as the State was obligated to pay it's budget every year.
I would wager with the ratio of kids to working adults now, and the significant increase in residential units and commercial real-estate/businesses moving in that the tax base for the City of Boston (which is based on property, not number of people) is higher than in the 1950s.