I mean... flexposts don't take getting hit occasionally gracefully. Wu is on record as saying they end up as "gross-looking crumpled plastic all over the place, because cars and trucks are running over the posts." If aesthetics are important to the admin, those will almost definitely take getting run over more gracefully.
Also, if the Google cache is accurate, the city posted a page about "Better Buffers" 4 days ago that evidently hasn't gotten any traction anywhere. As a replacement to flexposts, the city is apparently moving to cast-in-place concrete, built on top of the existing pavement without a full depth sidewalk foundation. This is evidently something they're also looking into for quick-build pedestrian refuges, and based on the promo image, one of the first places that will get it is Bolyston by the common.
We’re switching out materials to enhance safety for all travelers.
www.boston.gov