Non-metric systems used industries were chosen through experience and optimized in the same way that genetic algorithms optimize.
The SI units are rarely, if ever, more convenient than whatever legacy standard they replaced, but the path dependent development of naturally defined measurement systems meant different results in every country. Shipbuilding, lumber, textiles, printing presses would also end up with varying degrees of compatibility or difficulty in working with materials sized in another domain.
As an aside, the same people extolling the value of the metric system's base ten nature have little problem explaining why the dimensions of A4 paper are not round numbers. The legacy systems in their countries didn't come from users being unaware of a base number system, they just used to prioritize other conveniences that a measurement system can provide, such as how A0 through A8 paper sizes can be simultaneously cut from one sheet of paper.
Your post seems well considered, but I apologise that I'm struggling to ascertain the point you're trying to make.
I can comment on this bit though:
> As an aside, the same people extolling the value of the metric system's base ten nature have little problem explaining why the dimensions of A4 paper are not round numbers.
Metric isn't about round numbers -- this seems to be a common misunderstanding.
Metric users are happy to dive into sub-unit numbers (6.35mm is a number we see an awful lot).
Metric, I think, is mostly about being aligned with most of the world. A common language, as it were.
Secondly, it's about some pleasant relationships between different units (length, mass, velocity, volume, energy, etc.
Probably equal second, it's about making calculations easier.
Round numbers are irrelevant for this, of course. But base-10 is a massive boon.
I think the metric system facilitates trade, especially in bulk quantities of raw commodities, between different industries and jurisdictions. And, I think the decisions made to arrive at a universal standard are a combination of top down and "perfect" that typify nineteenth century science.
Earth doesn't have a 40,000km circumference, but it doesn't matter because anyone who needs a precise measurement will use some sort of aid. Similarly the mass of water in 1 mm of rain over a hectare isn't actually 10,000kg, but you'll use some aid if you need a more precise number taking temperature and actual contents of rainwater into account. Of course using nautical miles or acre-feet with high precision required aids of some kind, too.
However, legacy systems of measurements were developed to minimize the number of instances where people like machinists on a factory floor need an aid such as pen and paper to do a calculation. Big problems like Fermi estimates are much easier with the metric system, but day to day problems within a specialized profession probably are not.
I do think the benefits of a more universal system vastly outweigh the costs to convenience for everyday users of measurements, but I am skeptical of many of the rationales used for advocacy. Perhaps if an international system of measurements was designed today it would be even better designed for binary computers as computational aids rather than decimals and paper, though that would be an even more difficult transition.
The SI units are rarely, if ever, more convenient than whatever legacy standard they replaced, but the path dependent development of naturally defined measurement systems meant different results in every country. Shipbuilding, lumber, textiles, printing presses would also end up with varying degrees of compatibility or difficulty in working with materials sized in another domain.
As an aside, the same people extolling the value of the metric system's base ten nature have little problem explaining why the dimensions of A4 paper are not round numbers. The legacy systems in their countries didn't come from users being unaware of a base number system, they just used to prioritize other conveniences that a measurement system can provide, such as how A0 through A8 paper sizes can be simultaneously cut from one sheet of paper.