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> "Internal consistency" doesn't matter.

The lack of an opinionated stance on internal consistency is exactly how we arrive at traditional measures and also the US Customary set of units. It's far easier politically to be accommodating and allow more units than to put down your foot and say no, this is redundant, this cannot be used.

> Virtually nobody ever has to convert miles to yards

That's mostly true because it seems yards are only used to measure football fields and fabric. Everything else is measured in feet, from personal heights to furniture to rooms to house yards to building structures (the Empire State Building is 1454 feet tall).

But my point still stands. You think you don't have to convert between miles and feet? Okay: https://www.researchgate.net/figure/a-Typical-multi-lane-hig... . There's a highway exit coming up in 800 feet and another in 1/2 mile. How many times longer does it take to reach the second exit compared to the first exit? You have no clue. In metric it's 250 m and 800 m, and it obviously takes about 3 times longer to reach the second exit.

> These people never work with each other. This is a fantasy.

Tell me you haven't worked in engineering without telling me you haven't worked in engineering. If you eyeball everything and use intuition, I can see why you don't care about units, conversions, and calculations. If you actually need to plan and analyze things carefully before you order materials and cut things, you'll quickly see that having a plethora of units adds complexity and chances for error without adding any functionality that a pure system has (whether you're using millimetres or only decimal inches).

> But my point still stands. You think you don't have to convert between miles and feet? Okay: https://www.researchgate.net/figure/a-Typical-multi-lane-hig... . There's a highway exit coming up in 800 feet and another in 1/2 mile. How many times longer does it take to reach the second exit compared to the first exit? You have no clue. In metric it's 250 m and 800 m, and it obviously takes about 3 times longer to reach the second exit.

I’m having a hard time seeing this as much of a problem in daily life. That sounds more like a word problem in math class than something someone would want to typically calculate on the fly. And my sense of how far something at a given distance is in the car is informed more by experience and intuitive sense than measurement.

> That's mostly true because it seems yards are only used to measure football fields and fabric.

Also shooting ranges, but yes.

> There's a highway exit coming up in 800 feet and another in 1/2 mile. How many times longer does it take to reach the second exit compared to the first exit?

Prior to GPS navigation nobody ever said "there's a highway exit coming up in 800 feet"; road signs in the US consistently use fractions of a mile. When my GPS app switches units from fractions of a mile to feet, that means I can see where I need to exit/turn. Calculating a precise ratio doesn't matter. It's between 6 and 7 since there's 5280 feet in a mile and 86 is 48 but 87 is 56, but who cares?

Also, on a highway, I'm usually traveling at least 60 mph, and since there's 60 minutes in an hour, that comes out to one mile per minute. Try that with your fancy metric system!

> Tell me you haven't worked in engineering without telling me you haven't worked in engineering. If you eyeball everything and use intuition, I can see why you don't care about units, conversions, and calculations.

You're imagining a scenario where a guy who's worrying about fractions of an inch building a cabinet has to talk to the city planner who's worried about miles and they have to convert units to do that. That doesn't happen, and yet you want to optimize the entire unit of measurement for that specific use case at the expense of more common use cases like "dividing by three".

When it comes to domains where consistency does matter, we just don't do unit conversions. For instance, flight altitude is measured in feet, even when it's thousands of feet, instead of miles. If you're flying an airplane at 30,000 feet, who cares how many miles that is? That's not what miles are for. Likewise with domains that use feet per second rather than miles per hour. Again, even "metric" countries don't commit to the bit here; name me a country where the highway speed limit is in meters per second.

> When it comes to domains where consistency does matter, we just don't do unit conversions. For instance, flight altitude is measured in feet, even when it's thousands of feet, instead of miles. If you're flying an airplane at 30,000 feet, who cares how many miles that is?

Wrong. https://en.wikipedia.org/wiki/Gliding_flight#Glide_ratio

Your plane is up at 30000 feet and your engines are out. The nearest airport is 47 nautical miles out. The plane has a glide ratio of 15. Will you make it?

It's easy in metric: 9.1 km altitude, 87 km distance.

> Again, even "metric" countries don't commit to the bit here; name me a country where the highway speed limit is in meters per second.

I actually much prefer metres per second. It makes things like kinetic energy calculations easier. If I wanted to know the KE of a 1500 kg, 100 km/h car, I would first need to convert to m/s. Ditto the kilowatt-hour; it needs to die in favor of megajoules.

> Your plane is up at 30000 feet and your engines are out. The nearest airport is 47 nautical miles out. The plane has a glide ratio of 15. Will you make it?

Nautical miles and statute miles are different units anyway, so my initial assertion--that you would never convert flight altitude to statute miles--is still correct.

Glide ratio varies according to speed; the plane might have a glide ratio of 15 at one speed but a different glide ratio at a different speed. So in practice, the real world version of this word problem is more complex than you make it out to be, and pilots' handbooks will commonly have tables of glide distances to consult for this reason.

> If I wanted to know the KE of a 1500 kg, 100 km/h car, I would first need to convert to m/s.

You've adequately demonstrated that metric is more convenient for arbitrary word problems that you've provided. Real world applications is what I'm less convinced about.

> Also, on a highway, I'm usually traveling at least 60 mph, and since there's 60 minutes in an hour, that comes out to one mile per minute. Try that with your fancy metric system!

All right. I'm travelling along the highway at 120 kilometers per hour, which is 2 km per minute. Where ist the problem?

>why do we measure time in hours, days, weeks, months, and years rather than in decaseconds, hectoseconds, kiloseconds, megaseconds, etc.?

We don't use them generally because at the time of French Revolution, due to taxes, standardizing the state units of measurement of physical goods was a much more pressing concern than time. (If I were to guess, there were no work hours limits, and thus it just hadn't crossed the popular mind). Weeks were changed to have ten days though.

That's not to say that they hadn't tried: decimal time was mandatory for a few years before they realized that there were too many clocks around, and pushing decimal time can actually turn people hostile against the then-new metric system.

We still kinda sorta use them in form of fractions of Julian days.

>why do we measure time in hours, days, weeks, months, and years rather than in decaseconds, hectoseconds, kiloseconds, megaseconds, etc.?

Because it would be astronomically expensive to slow earth's rotation enough for a day to be 100 ksec.

There's no theoretical problem if we define 86400 seconds = 1000000 newseconds.

One big practical problem is that because SI is a coherent system, the second is embedded in many units. For example, 1 N = 1 kg m/s^2. 1 J = 1 N m. 1 V = 1 J/C. 1 Pa = 1 N/m^2. And so on and so forth. So all of those units will have to be replaced with units derived from the newsecond. This is kind of like how SI has the unit tesla but the CGS electromagnetic unit is gauss.

A long-term problem is that no matter what, the length of the day on Earth will drift from scientifically accurate atomic time. Sometime in the near future, a day will be 86401 seconds. Then 86402, and so on. So the old second or the new second will not solve this problem.

> A long-term problem is that no matter what, the length of the day on Earth will drift from scientifically accurate atomic time. Sometime in the near future, a day will be 86401 seconds. Then 86402, and so on. So the old second or the new second will not solve this problem.

How long term is this problem? For instance, we habitually insert "leap seconds" in order to keep things lined up, but if we didn't insert leap seconds, we might accumulate an error of maybe half an hour between the solar meridian and "noon" in the next 500 years, which is less error than we introduce by putting Madrid and Belgrade in the same time zone. And in 500 years, most of humanity will not even be living on the Earth anyway.