From John Bazell “In metric, one milliliter of water occupies one cubic centimeter, weighs one gram, and requires one calorie of energy to heat up by one degree centigrade—which is 1 percent of the difference between its freezing point and its boiling point. An amount of hydrogen weighing the same amount has exactly one mole of atoms in it. Whereas in the American system, the answer to ‘How much energy does it take to boil a room-temperature gallon of water?’ is ‘Go fuck yourself,’ because you can’t directly relate any of those quantities.”
The calorie used to be the base unit, until we released in the 19th century "wait, heat isn't a gas" and threw out caloric theory, and made the joule. Now the calorie is defined as 4.184 joules.
There have been multiple iterations of the "metric system" since it's introduction in 1792–1795, most notably the original 1795 draft variant, then the CGS (Centimeter-Gram-Second) version, then the MKS (meter-kilogram-second) variant, with the most recent incarnation being the International System of Units (SI).
That's why there are plenty of metric units, but not all of them are SI units. :)
Edit: Changed "1892–1895" to "1792–1795". Lol, whoops.
There's a popular argument against religion that essentially says that if any trace of a specific religion were wiped off the face of the Earth, it would never come back. As in there'd probably be something in its place, but there'd be no way that the specific beliefs practiced by that religion would ever return. Whereas if a piece of scientific knowledge were similarly wiped from human knowledge, it would eventually be rediscovered.
A similar argument can be made with the metric system: I think that if standardized measurement systems disappeared from the face of the Earth today, something extremely similar would eventually be invented and adopted. It's just too internally consistent and human mental math too grounded in decimal for it not to be. You'd probably even end up with a prefix-based (probably even Greek) naming scheme.
Now consider USC: the units fail to fit together in basically any meaningful way. They try but fail to be base-2, so you can't even come at it from the already-tenuous angle of base-2 being better than base-10 (e.g. volume skips what two quarts would be, weight is more like base-16 (???), and distance just does something so insane that probably 95% of American adults couldn't tell you how many feet there are in a mile). There are dozens of completely arbitrary, unintuitive, antiquated-sounding names (e.g. "horsepower"). Although the bases for metric measurements are rather arbitrary, they are extraordinarily precise, so much so that USC bases its own measurements off of insane but precise multiples of metric units. That's not to say that humans would jump straight to metric or anything, but moreso that whatever would fill USC's role as an intermediary between nothing and the metric-like system would likey be unrecognizable from current USC.
The "intuitiveness" of imperial measurements is that they're sorta human-scaled, at least for human-sized measurements. An inch is about the same length as the tip of my thumb, a foot is about as big as my foot, a yard is a single pace if I stretch a bit, etc. which makes it easier for a person to picture it.
Once you get out of that scale it really starts to break down though.
Yeah, the thing is that I actually don't terribly mind feet, cups, etc. as individual measurements. Taken alone, they feel intuitive. The first of two main issues is, as you mentioned, scale. And you could make the argument that you could just take the base units like feet, cups, etc. and decimalize them with prefixes. And that does alleviate a ton of problems with USC, but you also then run back into the issue of unit intercompatibility.
Metric units have quite an elegant and intuitive interplay that USC simply lacks. What's a liter? Why it's a cubic meter. But now if I try to relate USC volumes to cubic USC distance measures, things quickly fall apart. A gallon is 231 cubic inches exactly, which is a whole number, sure, but that's terrible for intution and for scaling. What's a kilogram? Why it's the mass of a liter of water. If you try relating pounds to units of volume, you might settle on one pound per pint, but that isn't true, as it's actually 1.041 pounds. So while it works at that scale, it quickly begins to fall apart and becomes completely inintuitive.
So unfortunately, even if we introduced intra-unit scaling by choosing one base unit and scaling that, working with USC would still be a nightmare when trying to intuit between different units. And this is, of course, not including things like horsepower that may have been intuitive 150 years ago but now are almost exclusively used 1) at minimum in the hundreds and 2) by people who have literally no concept of how much a horse can turn a mill wheel.
I would agree with you that something similar to metric would eventually arise, but I would consider duodecimal to make more sense than decimal, as 12 is a superior highly composite number and the terminating representation is much shorter for more commonly used fractions (e.g. 1⁄4 would be represented as 0.3, 1⁄3 as 0.4, 1⁄2 as 0.6, etc). I would also argue that groupings in powers of 12² make more sense than 10³.
I would also argue that it would make more sense for measurements to be based on natural units (such as Planck length) for all the basic measurements (second, metre, kilogram, ampere, kelvin, mole, and candela), such that the anthropic unit (the one you'd most commonly refer to without prefixes) would be some multiple of 12 away from the natural unit.
my pool 4.5m wide, 9m long and 1.5m deep, the current level of salt is 2.5g/l and a bag of salt weight 20kg. How many bags of salt do I need to bring the level to 3g/l ?
OR: my pool 14'9" wide, 29'6" long and 5' deep, the current level of salt is 2500ppm and a bag of salt weights 40lbs. How many bags of salt do I need to bring the level to 3000ppm ?
The answer to one is 1.5 bag, the answer to the other one is "fuck that, I'm getting 8 bags at the store and it should be good enough"
... weighs one gram ... An amount of hydrogen weighing the same amount has exactly one mole of atoms in it.
Not only was this never true - the sentence would have to have say "An amount of carbon-12 atoms weighing 12 times this amount has exactly 1 mole atoms in it" (far less elegant) -- but not even this is true any longer after the fuckup in redefining the mole in 2019, after which all these relations between amount of substance and mass are only approximate.
Nope, this redefinition isn't necessary, it is a choice SI made. Nothing would have broken by keeping an exact relationship between amount of substance and mass, it would just have retained the interpretation of Avogadro's constant from before 2019 (experimentally determined vs a defined constant).