Testing and validation are very important, but they're no replacement for structurally making mistakes as impossible as possible to make in the first place. In fact, that was the conclusion from the Gimli Glider incident, that using mixed units increases the likelihood of mistakes being made, and so they stopped doing that. It's kind of absurd to acknowledge that people make mistakes and therefore their work needs to be validated, but when the people doing the validation also make mistakes, they get all of the blame even when the people who made the thing did things in a way that increased their chances of making mistakes when they could have chosen not to.

Also, that's some contrived scenario you're painting.You make it sound as though every machine shop in the US would have to replace all of their equipment. First of all, for anything computer-controlled the units are arbitrary and software-defined. But even for purely (electro-)mechanical machines, it's not like those can't be (and aren't already) modded up the wazoo. Why replace the entire machine when you can just swap out some of the gears or even just the dial? If a machine has been around since 1945, they'll have done things like that many times already.

Of course no transition is going to be instant or painless, but it's better than keeping up this situation forever. I mentioned two incidents because they're the most dramatic, but things like that happen every day and the cost of lesser incidents also builds up. Somehow, almost all of the rest of the world managed to go against centuries if not millennia of tradition and momentum and transition in a fairly short amount of time during a period when precision engineering was already a thing that happened at a large scale, but the US is special? Give me a break.

Metric is a relatively recent invention. Every single country has had to ditch their measuring equipment in order to convert to metric.

The reason it's so expensive in the US is because they refused to change for so long. As the economy grows, the amount of equipment grows, so the more time you take to switch systems, the more expensive it will be.

If the US wanted to switch relatively painlessly, they'd just gradually make some measurements officially in metric. You can produce products labeled in metric with imperial tools. If those tools are precise enough, or the error margins big enough, you would not notice the difference between imperial tools and metric ones. In fact, there's probably many tools out there that can produce in both systems.

It's not a simple problem. But if it never starts to get solved, it will never be solved.

The main obstacle is not economic, it's cultural. People in the US are used to using imperial, many only used metric at school. They advocate online that their system is better, and keep using it.

If USAians used metric in day-to-day life, they would prefer consuming products with metric info on them, so companies would produce more metric products, so they would have more incentives to adapt their tools to metric.

How do you change culture? Simple. The first step is to make something "official". Making metric official and "deprecating" imperial would mean that communication with the government would be in metric. Laws would have metric measurements, technical documents provided by the government would have metric measurements.

If someone wanted to use imperial, they would have to constantly convert the numbers of the government, and you can't ignore the government. So you either stubbornly convert each time, or you give up and start using metric yourselves.

Of course in the early stages, the government could provide imperial measurements too, as a sidenote, a footnote or an appendix. But the main content would be in metric.

But the US has decided it doesn't want to do that. I don't want to get too geopolitical, but that only works while the US is an economic superpower. We'll see what happens once that is no longer the case.

In the meantime, all I can do from the outside is try to convince random people from the internet. If they don't switch to metric, they might at least stop advocating for imperial, and that would be a win.