The podium of Echelon (Parcels M1/M2) is rendered as a high end outdoor mall in an era where malls & retail are dying. Heck, Newbury St can't even sustain retail anymore.
The retail downturn is more specific to suburban big-box type stores. Smaller-scale retail in thriving urban cores is by and large doing fine. Bars, restaurants, convenience stores/pharmacies, and grocery stores will never go away.
We on archBoston frequently lament the fact that dive bars and clubs across the city are being priced out. "Dive bars are being priced out" and "retail is dying" are not compatible narratives.
Newbury St's issues are overblown, and to the extent that the street does have a decent amount of vacancy it's due to unsustainable asking rents. Usually when a street has a hard time filling its retail spaces in today's climate it's because of the landlord, not the market (see: Bromfield St, the block of Boylston east of Berkeley, etc.). Where landlords are committed to filling the space, they do (see: 101 Seaport, the Harlo, all of Assembly Row, One Seaport Square, etc.). This also goes for Greater Boston's traditional squares. Places like Harvard Square, Davis Square, Coolidge Corner, etc. have vacancy right around zero.
But yeah, at a high level, I agree with your take. Not all retail can be glitzy high-end stuff at $100 psf. With as much retail coming on line as we're seeing in the Seaport, asking rents will have to drop and tenant diversity will increase. That's a good thing.