A few personal opinions about this debate:
1. Let's make sure we're asking the right questions when talking about preservation. "Preservation" is not - or shouldn't be - an end in and of itself. I see little point in preservation solely for aesthetic reasons, there's needs to be a functional component. If we want to retain the visual urban form (which in my opinion more than anything else, endows Boston with it's 'attitude' or 'Boston-ness'), then we need to make sure that these buildings can adapt to changing social and economic forces. That means warehouse conversions to office/residential space, interior work, energy efficient retrofitting, perhaps small alterations (very small) to street-level facades that allow for retail space.
And there are good and examples of this in Boston. The Russia/Atlantic Wharf development is, in my opinion, an excellent project. Looks good, gets the proportions rights, creates a stark dichotomy between old and new but retains some (but not superfluous - a good balance) homages (thinking the "warehouse"-esque window bays) to the older structure. Then there's 125 Summer St, which I think is a cautionary tale. It overpowers the environment created by the 19th century commercial blocks, rendering the whole area with some weirdo-disneyland vibe.
The question in my eyes then, is how do we instigate more of Russia Wharf and avoid 125 Summer. Instead of rebuilding or preserving, it's solely a question of how best to balance the two goals, because let's be honest we need both.
2. What's our recent history. In the late 1940s as residents fled Boston, some businesses bandied about suburban relocation while others closed up shop entirely, the City compensated through steep property taxes. Apartment houses were often assessed at 60-80% market value, commercial blocks at 70-90% (some even assessed above market value), threedeckers and other multi-families around 40-50%, with single-fam detached homes clocking in closer to 20-25% of market value. The anti-Curley reform movement assumed power through attacking these woefully unbalanced conditions, but their coalition deteriorated when the question shifted to "now what". There was a municipal fiscal crisis that needed to be met, but residential property owners didn't want their rates to rise and blocked attempts to do so. The Hynes coalition tried to employ an early form of what would become the Pru 121A bill, basically allowing the city to offer tax breaks for new development with lower assessment ratios on those new buildings. They were rebuffed by the SJC because that available tax break code placed a requirement that the new structure bring "public benefits". Thus, for most of the 1950s, the only way to instigate new development and lower assessment rates was through urban renewal powers.
Owners of older properties were not offered tax relief and the high rates persisted well into the 1970s. Without the relief, little investment occurred in these buildings, which deteriorated lowering their market value, which further exacerbated the incentive to refrain from investing as did other macroeconomic influences and in select cases, the threat of demolition by the BRA or DPW for urban renewal/highway plans. City leaders were eventually able to get the Pru bill passed allowing for tax breaks on new developments, but the high assessment rates on the older properties remained. "Superblocks" were preferred over small, irregular lot sizes because the belief was that the smaller the lot size, the less economic incentive for a property owner to invest in improvements. In fact, by the mid-1970s, the economy of Boston and it's construction market had improved significantly (prior to the "Mass Miracle", the recovery was focused on the older FIRE industries), but the financial crises had not abated as much of the new development received tax concessions unavailable to most property owners in the city, which continued to suffer under a rather oppressive tax regime.
The lesson I see in (my reading of) this period is that we can't talk in absolutes about the economic ineptitude of older building stock, much of conditions were created by human policy, social/cultural attitudes, not unchangeable "market forces" though they certainly played a substantial role
3. It's not just about Bulfinch and bricks. The building to be razed in this project is a latter-day, early 20th century commercial building. I don't see what context it contributes to honestly; that end of Broad St was thwacked by the Central Artery and the Granite warehouses on the opposite side of the street are far more contextual buildings than this one. It's not that it isn't worth preserving but is it worth the struggle? I don't think so. Pick and choose battles; cooperate with developers where necessary, that more than anything is going to produce the results and preserve the environment that we want.
There's also a lot in Boston beyond Bulfinch and beyond 4-story brick commercial blocks. Boston has been constantly redeveloping and, if you ask me, the best buildings we have are from the 1875-1910 era that encompassed both technological changes in construction, Romanesque, high vic, and other historicists styles that I think are really striking. They don't seem to fit the narrative of a lot of the early preservationist in Boston; Whitehill for example is laudatory of nearly everything built by Bulfinch, laudatory of the Second Empire/French styles, laudatory of the buildings built when Boston was a grand old city, but barely mentions H.H. Richardson and has general dislike for "cheap copies" (as many medievalists did - Whitehill was, by trade, a scholar of medieval architecture) produced in the later 19th century, saving praising mostly for monumental works like the BPL library. Those 4-story brick buildings are nice, but there's plenty others along Stuart/Kneeland that I wouldn't be too sad to see redeveloped. I think we forget that Boston is structurally, a late nineteenth-century city - one with a distinct environment in the US, but not as a Colonial city in regards to it's building stock, more of a 19th century city overlaid on colonial contours.