I wonder if doing raised planters on Stephen ave would work. Can incorporate benches or a spot for food vendors maybe. Just keeping the salt away from the trees would make a huge difference!
I am not a huge fan of raised planters in areas that are supposed to be major pedestrian areas. They just seem like they create too much of a barrier/obstacle. Barclay Parade is a good example (or maybe a bad one, given the size of the planters they have). I get and support the raised planters along the median of roads like Memorial and 16th Ave, but my first thought is I don't want them on Stephen Avenue.
From what I recall on previous discussion around street trees is that the main challenge is the root systems. Surprisingly - small concrete sarcophaguses are not a good medium to grow healthy roots. Some of the better approaches are those plastic soil cells so the tree routes can go under the pavement, but also just making far more permeable areas for water to seep through. Doesn't have to be garden beds/planters necessarily, but just needs way more surface area where roots exist and access for water to enter the ground. Stephen Ave could easily have permeable surfaces that support foot traffic. I can imagine some stupid rule exists that a pedestrian street needs to be able to support a 25-tonne fire truck for reasons and therefore a road department would reject any permeable surfaces that can't support that weight as the fire truck would be prioritized.
The second part is the maintenance and replacement - small, transplant trees struggle as it is, more so if they aren't watered or prunes properly in the early years. Lots of issues with doing this consistently or adapting watering schedules because of a heat wave. Dead trees aren't always replaced - sometimes because it's learned that the tree will never survive anyways (concrete sarcophaguses), but other times because it seems like it's no ones job to even try.
The background problem is the general environment - windswept corridors between buildings, high amounts of salt, drunk guys ripping off branches of young trees, annual mega-hail storms, drunk drivers that plow into trees - these are all factors that make a hard job (i.e. to grow a tree in a highly variable high-altitude prairie climate) just that much harder because the tree is on the street. So if we want trees, there should always be a bit more attention to detail required to mitigate some of these general environment risks. If we don't put in the extra effort, the trees will often still fail.