I think theres also a fairly easy happy medium path that should be taken first before we even need to think about bulldozing low density neighborhoods. The first would be as mentioned, adding lots of density abutting highways, on old industrial sites, parking lots, and empty lots (especially around transit). Theres still far too many empty/underutilized lots across the city that could be dense housing.
After that theres hundreds/thousands of single story retail buildings lining roads across the entire metro area along with strip malls and other single use buildings. Adding 3-5 stories of housing above these while maintaining the ground floor retail would be another one of the easiest ways to infill lots of density without displacing ppl.
Heres an example right on Beacon st where theres a strip of single story retail buildings flanked by a 4 story residential building on one side and a 3 story residential on the other.
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Buildings like these exist on every main road in Boston and hundreds/thousands of other roads. Theres no reason you couldnt have a 4 story residential building here and have the exact same retail situation at ground level. Adding housing above all of these buildings would keep the feel of the city the same while adding thousands of new units of housing.
Heres another on commonwealth, theyre everywhere.
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