"By the same logic as Chesterton..."

Why? Are they the same problems? The problems of not having a fence vs. having a fence? Are the false positives and false negatives symmetrical? That would be a massive claim! (Normally, by risk mitigation, they are asymmetrical; the unfortunate reason why we have anxiety in life, and that stupid smoke alarm sound). 

Building a prior fence: Maybe the fence was taken down, cause there was no risk of flooding, and it was blocking the animals? Do we have a reason to build now or not? 

Chesterton's Fence: We should not take a fence down, because maybe there will be flooding? Or it will not offer protection from an invading army, and everyone will die? We don't know, lets not risk it. 

There NOT being a fence, is not evidence, that there shouldn't be a fence (even knowing there was or was not a fence there)...go out and see, should there be a fence or not?  

One action requires refraining from destroying a possible benefit; the other leads to action of building a fence. You always normally need a good reason to build a wall. There having already been a wall there helps, but is not dispositive by logic, because as you say, the wall is down, and we have to speculate why it was there or why we should build it now.  

The upsides and downsides are very different in your scenarios. 

Well, I have to think there is some balancing act here. I do look at life a lot from an evolutionary standpoint, but a flock who listened to their leader, and he was 99% wrong, would not survive long; or the sequence has to start with crushing a homer, and then getting it wrong 99 times. Whats missing here is not fully setting forth the downside of one of the 99 bad ideas.

Or maybe because we survived the basic-needs Malthusian "filter"; that explains the "Moral Outrage"; possibly just outraged at too much information and supposedly "true" memes, and the ratio keeps getting worse. A market crash of the idea ratio. (stagnation or plucked low hanging fruit)

In the end, if you hew to pragmatism (survival and reproduction), you guarantee the Great idea/ to Bad Idea ratio is relatively solid and tested. The theory is we want stability, which just a touch of creativity. Society also allocates places with high-risk reward ratio...and we give them a PHD.