As you consider "what if?" options that relate to the sidewalks and retail storefront options on Tremont, bear in mind that retail is as low-key as it is because that's the way that ToC residents want it to be. Historically, the ToC condo association has kicked up a fuss about any potential usage in their building that they believe would create trash or noise or require off-hours deliveries, etc. This most definitely includes restaurants and any retail tenant that might have late hours. In fact, they've been pretty vocal about retail uses they don't like even a few blocks away. Historically, their priorities have been pretty much mirrored those of Charles River Park residents. (An additional problem with anti-urban bunker-towers-in-park of the '50s and '60s is that they were marketed to and appeal to residents with a fundamentally suburban mindset, and that mindset may ultimately be more street deadening than the architecture itself).
Has their headset changed? If not, you might be surprised to find that your good idea to expand the sidewalk and encourage more streetlife there, even if fully funded, will be greeted with a chorus of disapproval. If the interest of the building owner is purely economic, then there is often some effort to mitigate the Corbusierian sins of the architectural past: see, for example, incremental efforts of the Radisson hotel, a few blocks away, to try to wrestle with the facade of their concrete bunker so that they can actually monetize their streetfront. That hasn't been a priority for ToC, and that's a big reason why the limited hours hat shop and the shoe repair shop are there and the effort spent on spiffing up the storefronts has been minimal.
