The trouble is there is just very little gain. It really just doesn't matter. All the systems are fine, they all work. If you come live here, you'll adjust after 2 years. If I moved to Europe, I would adjust in 2 years. Once in a blue moon you have to bother with converting units but c'est la vie. There's bigger things to worry about.
Two cups to a pint, two pints to a quart, four quarts to a gallon. That makes sixteen cups to a gallon. There are 5280 feet in a mile, and three feet to a yard, so that makes 1760 yards to a mile (if I did the math correctly in my head just now).
These are conversions I know off the top of my head, I didn't need to look them up. Which is the point the GP was making: it's not hard to memorize the handful of conversions you will encounter in everyday life. Most people living here did it as children and have never had to think about it since. That's why there's no actual gain for us to switch to metric units. On the other hand there would be quite a bit of pain as everyone had to adjust to estimating things in kilos vs pounds, grams vs cups (in recipes), and so on. So for the typical American, it is actually a net negative for the country to switch to the metric system. It isn't just stubbornness.
Yeah, every system has pros & cons. I think the lack of an approximately-one-foot (30 cm) unit in metric is clumsy to work around, and I think degrees-C are too wide. We can argue about the details if you find it fun ("yards in a mile" does not come up all the time), but they're all evolved from hundreds of years of usage, and that means they all work fine at the end of the day.
> I think the lack of an approximately-one-foot (30 cm) unit in metric is clumsy to work around
What's clumsy about 30cm though? If you are working at scales where this level of precision is needed, you can just use cm throughout, and the beauty of metric is that even someone who has never had to do that before will know immediately how much it is because conversion to meters (or millimeters, or whatever the primary unit is in their usual applications) is so easy.
Similarly, I've heard similar sentiments expressed about lack of pound equivalent in metric. But in practice we just say "500 grams" etc (and for bonus points you get 400 grams, 300 grams etc).
Miles and yards are both used as units of distance, so conversion is obviously relevant. The only reason why "yards in a mile" doesn't come up all the time is because Americans work around it by subconsciously (?) avoiding any such cases where the conversion is non-trivial. E.g. a road sign in Europe might say "400 m", whereas in US a similar one will be "1/4 miles".
And "evolved from hundreds years of usage" generally means a lack of internal consistency, because most units originated a long time ago as a way to measure something very specific - in many cases, something completely irrelevant to most people using those units today. Nor did those units remain consistent through history - just look at how many definitions ounce has in US in different contexts, all of them historical! Or regular vs nautical vs survey mile. Even just cleaning up that mess would be a massive improvement.
> would be a massive improvement
This is where we disagree. It would be a small improvement at best. Most of what you're pointing out are the awkward corner cases that just don't come up or, like you said, we already have other solutions for. Outside of some specialties, pretty much no one needs to know how many cups are in a gallon or yards in a mile or what a nautical mile is. I don't know those things, and I somehow get by OK.
I've been living in US for 15 years now and I still can't remember which unit scales are factor-of-3 and which ones are factor-of-4. How many cups are there in a gallon? How many yards in a mile? I don't want to waste my brain cells on stuff like this, yet it comes up all the time in e.g. cooking, or using maps for navigation.