Since we're talking about an extension that'd dump most of it's riders into Boston proper, it's not (at least not initially) about sheer density, but the density of commuters connected to Boston. That's why the South Shore Ext was necessary, probably moreso than Lynn at the time of project execution.
Quincy is about as quintessential a railroad town as there is thanks to the Old Colony. It's fortunes have always been tied to Boston. Despite the industry that exists down there, there's still a far, far, far more direct socio-economic connection between Boston and Quincy than between Boston and Lynn.
And that makes sense. The North Shore has significantly more heritage density than the former areas of the Plymouth Colony, modern-day Essex County was, from the 1626 to the industrial revolution, the cite of the majority of Massachusetts' cities. Just prior to the industrial revolution in 1820 (aka the immediately "Pre-Lowell Era"), the North Shore towns hugging the coast from Newburyport to Lynn accounted for about 55k or ~10% of Mass' population, Boston and it's surrounding towns had just over 60k (12%), while the South Shore towns in Norfolk and Plymouth Counties mustered between 30-40k depending upon how expansive you make. These are census stats, fwiw, they inlcude towns later annexed and include towns that have since become independent. It's not the most precise measure, but there's an important point there - the North Shore was always subject to far more economic pulls radiating out from it's early cities. Those pulls would only increase in strength and in number with industrialization of places like Lynn and the Merrimack River.
Quincy residents, as Van says, would've piled themselves into the Old Colony, which was for many years the railroad with one of the highest riderships oriented towards Boston. It very much guided the developed of Quincy as a town. Lynn, in contrast, also had close railroad connections, but also sat at the center of it's own commuting catchment area serviced by the Eastern Mass/Bay State system, the Eastern, and the Narrow Gauge which, as it didn't traffic in freight, operated at near-rapid transit levels of service and all-but-built the town of Revere. Come deindustrialization and the collapse of the railroads, Lynn's traditional catchment area was primed to be stripped off from the urban economy by Route 128 industries, which were initially - and are to this day, as I'm sure Westie will remind us - located primarily in the arc from Waltham to Burlington. Quincy was not a 128 town, the South Shore developed mostly as bedroom-suburbia with it's residents partially connected to the old industrial centers, but also substantial by former Bostonians decamping for that wholesome lawn-chilling, kid-raising atmosphere. But it was still connected to Boston, without the OC these commuter were driving in over the Southeast Ex which was the most congested road entering Boston (excl the CA) by 1970. Lynn didn't have the same highway connections, so in losing it's economic pull, the transition from former urban center to a satellite of Boston was less assured. That's what I'd say laid the foundation for the South Shore Branch: closer connection of residents to Boston, congestion-relief opportunities, greater immediate need, rather than sheer density.
And it's all of that that I think makes Blue-Lynn interesting insofar as there's really high ceiling for success. Lynn, like the other old industrial stalwarts of Cambridge, Boston, Lawrence, and Lowell has grown only marginally since 1900 (granted, that's a linear statistic - it's been very up and down, for sure). Quincy was 35% of the size of Lynn in 1900, now it's slightly larger. Other former industrial towns switched up and caught a lot of the people fleeing places like Boston with suburban development - places like Brockton and Framingham and Waltham - all of which are more than twice their size 100 years ago, despite being industrial centers and subject to similar economic decline as Lynn. As Boston and Cambridge go their shit back together they basically scooped the detritus of Lynn's old catchment areas and re-centered the travel patterns on the urban core in the past 25 years. But Lynn is still there, is still substantial, but is almost bizarrely unconnected to the economic forces around it.
The number of jobs in Lynn has declined 25% in the last 25 years (about 8k in real value), which is greater loss than any city around Boston, the next steepest decline in Lowell comes in at about a loss of 4000. Meanwhile the nearby Everetts, Maldens, Peabodys, and Beverlys of the area had been registering steady gains over the same period. Lynn is still only marginally connected to Boston - we're talking roughly 16% of it's workforce that commutes to either Boston or Cambridge daily, compared to over 40% for town like Quincy and 30-50% for towns like Revere, Medford, Everett, Malden. So the increasing sway of Boston seems to just be bypassing Lynn - the daytime workforce that Lynn has managed to keep still draws in greater proportion than any town in metro Boston (excl: Boston and Cambridge) upon Lynn-resident workers. Roughly 50% of the jobs are filled by people living in Lynn, 50% are filled by people living outside the town. For comparison, 78% of Cambridge's daytime workforce originates outside the city, 61% for Lowell, 83% for Malden - not all represent economic pulls of course, but they do indicate that there's some cross-boundary movement, which Lynn should, in theory, see far more off than it does in practice. There are three types of towns in Mass that have such meagre economic pulls - very bedroom-y suburbs (think Scituate), isolated areas (think Nantucket), and the hulks of former industrial centers (Lynn is most comparable to New Bedford, Fall River, and Pittsfield in this regard - expect all of those areas are farther removed from the economic engine of Boston).
So the upshot for something like Blue-Lynn is that, there's this substantial city lying just up the way that is shedding it's job base and it's residents are slowly turning to Boston as a place of employment, but it still lags far behind nearby areas. And that's the pitfall too as I see it, I can't really think of another example of major former industrial town being linked up to the rapid transit system since the early 1900s (with the maybe exception of Everett and Malden) - and even then, places like Cambridge, Roxbury, Charlestown, and Dorchester had already become enmeshed with Boston's economy. I don't think it's a deal-breaker at all (in contrast, I think it's a reason to do it), but it is something to consider going forward.
