This is largely true. Also important to acknowledge that 17th Avenue is the only Main Street that's anywhere close to a local population and density to support itself at all. Between the Beltline, LMR, Cliff Bungalow, Mission you have about 35,000 people. Add in low density Upper Mount Royal, Bankview and Sunalta and we end up with ~45,000 nearby-ish at densities ranging from 8,000 - 12,000 people /km2.
But it's a bit catch-22: we only claim we can't support robust pedestrian infrastructure because we believe the street relies on suburban car traffic to survive. Therefore we have already started our path dependence based on an assumption. We can't expand sidewalks permanently or make them consistent - must preserve parking and fund road capacity upgrades instead to get more people to drive down. Pop-up anything must take over pedestrian space - can't touch through-traffic. All this is only based on assumptions, not how people actually get to the 17th nor how they get around when they are down here.
And about that infrastructure - I would definitely support pockets of a fully European-style car-free, pedestrian realm in our city. Unlike Stephen Avenue, 17th Avenue actually has significant density nearby to drive daily traffic. Such a move would be ultra attractive and boost the area to the next level as a place to visit and live.
But that's not what I am talking about. What I really am asking for is sidewalks to be considered at all.
The sidewalks of 17th Avenue are probably the highest pedestrian traffic streets in the city (I can't confirm because we don't count) and they are in also in garbage shape. They have random widths usually too narrow for moderate (let alone peak) traffic, and always covered in debris and signage - and that's before we threw a bunch of pop-up patios and construction fencing on them hap hazardly. What was an inaccessible mess to anyone in a wheelchair, with a stroller or carrying larger objects is now a completely unusable street, all because pedestrians and sidewalks have no practical priority in how the street is used or designed.
You shouldn't need 20,000 people /km2 to "deserve" a consistent, usable sidewalk width on a destination retail street.