I agree for the most part. It’s better to design streets so that drivers feel uncomfortable at higher speeds. Street narrowing, bollards, trees, smaller set backs can all slow cars down.
That said, as a counterpoint, despite similar street design, the speed limit in most of Canada is 50kmh (30 mph) and many urban residential streets are down to 30km (18.6 mph). Some people speed, but driving 45 mph (72 km/h!) down most city streets is pretty rare. Pedestrian and cycling accidents involving cars in Canada are close to half the rate of the US. Which is to say, I don’t think the Denver mayor’s proposal is crazy. It works in Canada, but it will take time for the culture to change.