There are too many leaps of abstraction, which to me, proves the missing horseshoe nail is irrelevant in the big picture. Too many other things could have transpired positively for the kingdom in a space so expansive. It’s classic scapegoating. “Bro my controller totally didn’t work that time! We would’ve won the match otherwise I promise.”

It's an ancient proverb demonstrating early understanding of complex systems. Not an in depth philosophical argument.

However there are plenty of real life examples of a single small detail causing outsize impact. See: https://en.wikipedia.org/wiki/Alaska_Airlines_Flight_261

It's kind of absurd to think otherwise.

Tbh it seems entirely plausible to me that a messenger being unable to deliver an important message could have an outsized effect on that outcome of a battle. What if they're letting their side know about a surprise attack?

A more robust treatment of risk factors in both ideas.

You want to ask whether the system needs to be tracking nail quality if the kingdom relies on nails that much. You also want to be asking why critical information is being sent by only one messenger.