alt Hacker News"historically rare high-trust"? You are wrong.
Most self-forming human societies are high-trust and more loyal to each other than to truth (as the models have so eloquently regurgitated). Look anywhere from the hunter-gatherer tribes to villages to small towns. I have traveled my small, poor country extensively and the only place the fundamental idea of altruism had to be "consciously invented, built and protected" was in the big cities. In the rest of the places, it came to them as naturally as hunger. We've found skeletons from thousands of years ago that indicate that the whole society came together to care for the disabled individual for nothing in return, leading them to live as long as a modern disabled person with healthcare might.
Societies become low trust driven by specific factors: resource scarcity, something forcing a lot of different groups of people into the same area without integration. This is not a defense of humanity. I'd say the case of the returned wallet is more an example of class-loyalty than general human altruism. Humans have an instinct for exclusion of the non-conforming.
But the idea that civilization has to be invented and maintained is a debunked, regressive, even dangerous idea in that this line of thinking was used to justify a lot of atrocities historically.
The argument that we are historically high trust in small tribes, is not a denouncement of the position that we are in a unique moment in history.
Altruism, as you pointed out, is conditional. If population grows, the new constraints change the conditions that determine societal behaviors.
We need to develop tools to maintain a social contract once we grow past a degree of complexity.
We may be recreating the things we value from the small group stage, however our ability to achieve that in the recent past was an achievement.
It took a lot to get there, and tons of ideas and counter arguments were attempted and their failures learnt from.
Even the past moment had huge flaws.