The comfortable temp for biking is significantly higher than it is for walking, especially with the right gear. 40°C is definitely beyond reasonable tho. Planting trees and decreasing the amount of asphalt would go a long way to make it a better proposition more of the year. A societal expectation that you don't go or do anything when weather gets that hot could bridge the difference. Unfortunately that kind of philosophy is antithetical to capitalism's demands for productivity.

Even if the city is flat as fuck you'll still arrive sweaty if the climate is hot. Take Phoenix for example, you will sweat even if you are in the shade and doing no physical exercise because it's commonly 46 degrees.

Biking in the cold and wet honestly isn't that bad. Biking is my primary way of getting around all year in the PNW. When it gets real cold I put on normal snow gear. It definitely makes going outside more of a production tho.

A lot of it has more to do with what people are used to and feel is reasonable than with the actual conditions. If people saw more folks riding and actually knew people who rode I think people would be more open to try it.

Unmanaged ice/snow, unhealthy wet bulb temperatures, and getting run over due to car first infrastructure are the most significant barriers to more people using bikes as transportation IMO. If a society chooses to, all those things can be mitigated.

My favorite part of riding is that I get moderate physical activity for free. I would not spend near as much time being active otherwise.