Great essay, Douglas. The path integral metaphor is beautiful and evocative (and among other things reminds me how much I loved Feynman's little book on QED). I completely agree with you on the advantages of social experimentation. I think Diamond is right to argue that Europe's fragmentation gave cities and states the space to experiment that eventually produced modern liberal democracy and capitalism.
You anticipated and addressed almost all the caveats that struck me. As you say, institutional structures may not scale up (or down--it's easier in many ways, for example, to address the entire US health care market at once than in pieces). And politics constrains what's possible, since we aren't governed by technocratic utility maximizers.
I would also add that while we may not have the ability to control which black-swan events befall us--this is what I write in my own essay--we can nevertheless make society more resilient to unexpected shocks. We can't prevent lightning from striking, but we can ensure ourselves against damage. In general, I think we should prepare not just for our median, best guess about the future, but for as much of the space of potential futures as possible.
Again, great essay. It deserves to do well.
Best,
Robert de Neufville