Your idea of zero-slack efficiency is interesting theoretically, and I haven't heard it articulated as well as you did. But I don't think it's valid in a practical sense, because you can't just ignore the cost of (say) missing a flight because you were a minute too late. If you ignore one side of the coin, you no longer have a valid model of efficiency.

Going back to your example of a flight that's canceled, the airline should account for it. One simple way is to multiply the cost of that event by its probability. (And multiply it 5x or something, since passengers will be angrier at having an hour of their time wasted than happier at having an hour saved.) When you look at it with this framework, it makes sense why a pilot doesn't skip his checklist because the cost is very high, even if it's a 1 in 1000 case he's trying to prevent. In other words, it's not just how often it occurs, as you suggested.

Another example is how Bill Gates, in the early days of Microsoft, kept a year's worth of salary in the bank at all times. And how just-in-time supply chains broke when the 2008 financial crisis hit. Another way I heard this stated is that the rich are not the people with the most money, but with the most options. Or, hypothetically, a company with 100 engineers who builds iOS and Android native apps dedicating one engineer to building a hybrid app in Flutter because wasting 1% of engineering is time is better than betting the entire company in a wrong direction, thus wasting 100 engineers' time, though the "plan B" engineer's work is unlikely to yield a launchable app.

You also rightly point out issues like short-termism and misaligned incentives, but those have been discussed many times already, so I'm not going there.