>while the children in the countryside were always playing in the mud, snow, trees, and so on, getting dirty, tired, and scraped.

Now. Expect someone to show up with some statistics of how accident rate in the country side is greater than the rate in the city, along with other folks with supporting argument like. "A weak kid is better than a dead kid", "Mental handicap is better than physical handicap", which are obviously not false!

All the evils of modern life can be summarized in a single word..."Fear".

The next breakthrough for humanity would have to be freeing the masses from being perpetually afraid! Might be a bit hard since the fear this fear can be capitalized for massive gains by the current establishments !

I see where you're coming from but I think there's also a lot of unacknowledged variables in play here that get glossed over because of a focus on the outcomes such as dead kid vs. live kid. For example, I'm wondering if the same rules and recreation structure that result in a dead city kid result in a dead rural kid. A lot of environmental factors including traffic, neighborhood crime, average physique (in areas where children might often participate in their parent's physical labor), etc. could all create outcomes that are generally more potentially lethal for a child raised in a densely populated metropolitan area than for a child raised in a rural area.

Or maybe the countryside kids have a better understanding of where real danger is from their experiences.

that's the biggest thing - you have to try and fail. Wisdom comes from learning from your past mistakes.