I think part of the reason roads are good is that, if more people get on bicycles, road wear decreases, and keeping roads in good shape becomes more affordable.

The fourth power law (https://en.wikipedia.org/wiki/Fourth_power_law: “The stress on the road increases in proportion to the fourth power of the axle load of the vehicle traveling on the road”) is very harsh for heavy vehicles.

Precisely. Almost everyone requires a car occasionally -- maybe when they're moving between apartments, or traveling to a hiking trailhead not accessible in any other way, or working a job that requires a large volume of equipment. By giving people other options when they don't need to use a car, those car trips become more bearable in many ways -- including infrastructure efficiencies.

I've heard of people in the US who needed a car just to go to a shop on the other side of the street. The street was a massive stroad with no way for pedestrians to cross, so the only way to the shop was to get in the car, drive, turn around, and stop at the other side of the road.