Neighborhood change comes through the people, not the buildings.
It is true that [A] cheaper "townie" (for lack of a better term) retail establishments are being replaced by higher-end retail establishments across Greater Boston. It is also true that [B] certain "taxpayer" retail buildings are being replaced by denser mixed buildings in certain locations. But it is not true that [B] results in or causes [A].
We see [A] all the time in neighborhoods where there is no [B]. Look across Somerville (e.g., Davis Square and Union Square) or Cambridge (e.g., Harvard Square or Central Square) or Brookline (e.g., Coolidge Corner or Washington Square or Brookline Village) or JP (e.g., Center Street). All of these places have seen plenty of "gentrification" (again, for lack of a better term) of their retail environments in the current century but relatively little change to the physical buildings that actually house the retail. This is because physical change trails population change. The same socioeconomic dynamics that push out low-rent mom-and-pops in favor of higher-rent bougie places (and low-income people in favor of high-income people) also make real estate development profitable. But that real estate development itself does not price out the mom-and-pops. The entire YIMBY economic philosophy is that the expansion of physical supply in in-demand areas can actually forestall population turnover, while regulations that prevent the expansion of supply accelerate population turnover in existing supply.
The South End itself is a great example of this! The heart of the South End (think the area roughly bound by Columbus to the NW, Berkeley to the NE, Albany to the SE, and Mass Ave to the SW) has actually seen very little new construction in the last half-century. You can point to a handful of relatively small scale projects here and there, but walk down any random street in that neighborhood and the vast majority of buildings (especially the residential ones) are the same buildings that were there during the Reagan Administration (or the Coolidge Administration or even earlier, save for the post-war public housing developments). But the overall "vibe" of the neighborhood, the retail environment, and the population demographics look nothing like 30 years ago. The buildings didn't change, the people did. And those new fancy people (the “gentry”) brought with them tastes and budgets for new fancy retail. Resisting change in physical buildings isn't going to forestall demographic change when those new demographics are simply moving into the existing physical buildings.
I hate that cheaper local retail is being priced out of Boston, and I 100% would support policy changes tailored to address this. But preventing increases in physical density is not one of these policy changes. What we need are policies such as 1) inclusionary zoning for retail (like we have for residential) that requires a certain % of new space to be leased to local tenants at below-market rates, and 2) state-level liquor license reform that allows more permits to be sold across Boston at prices more affordable for low-volume / lower-price establishments.
