I again agree with your sentiment, but disagree with the details you provide.
Here is a map of Quincy's census tracts by population density. Apologies for the quality. I outlined Quincy in maroon and added red stars where the T stations are:
View attachment 44747
10,000+ per sq mile is a good, loose starting point for "urban" or "deserve rapid transit" per this discussion. You'll notice that all of the relavent census tracts have a density per sq mile of 5k-10k or 10k+. The only reason Quincy comes in with a 6.5k per square mile density is becuase much of the Blue Hills Reservation lies within municipal limits, which I'm sure we can agree has nothing to do with assessing how much a neighborhood miles away deserves transit.
Here is Arlington:
View attachment 44748
You'll notice the relevant census tracts in Arlington have basically the same density as the relevant census tracts in Quincy. Really, the only difference is Blue Hills. In fact, if you remove the area west of I-93 in Quincy, which is mostly the state park and has none of the walksheds of the transit stations, the remainder of Quincy has a population density of about 9k/sq mile, very very similar to Arlington.