JimD said:
The industry I use to work in is nuclear power. The most common fuel rod diameter in the world is 0.374 inches which is 9.5mm. All our U. S. customers talked in inches and our European and Asian customers talked mm. We used fuel tubes from a European supplier and a U. S. supplier interchangably. We exchanged a lot of other parts too. When I say the units of measure are interchangable I am speaking in part on that experience. The consequences of significant errors could hardly be bigger than with nuclear power. There have been zero significant issues traced to units of measure (and unlike most industries, any error gets a root cause investigation).
All you are saying when you call U. S. standard measurements incoherent is that you find them illogical. I wouldn't use them either if that was true in my case. But you saying they are illogical (to you) doesn't affect my opinion, I prefer them.
Another reason everything has not changed is the lack of sufficient benefit to justify the cost. Drain lines for tubs are 2 inch and sinks and showers are 1.5 inches. Changing to metric means changing a bunch of building codes and dies to make the plastic tubing and fittings. When you are done, what benefit do you get to justify the cost? European suppliers can more easily supply? That obviously isn't getting us there. The same thing is true for plywood sheet sizes and lumber sizing. It could be changed but where is the benefit to justify the cost?
I don't think anyone is talking about going back to legacy systems and re-designing them. It's well known that many fields that have systems in use that span decades back aren't going to change legacy systems (and for good reason). Thus why aerospace hardware is still heavily inch, defense hardware same, with newer stuff metric. Nuclear power is another good example.
What is the point is get everyone switched over, so new stuff going forward is metric. Most places are going to follow the same basic rules, old stuff is inch, and everything done on it stays that way. New stuff is metric. Sometimes old stuff gets carried forward, it is what it is. Sometimes old stuff gets new stuff added to it, thus the new stuff may be done in inch, but also it may be metric because all the bits for the new stuff are metric. Just because stuff is old, doesn't mean it isn't metric (lots of folks get surprised when working on really old stuff to find it is metric, even long before the metric conversion days). Also just because it's old and in inches, doesn't mean support documentation/math wasn't done in metric.
The core issue is those who use old stuff as an excuse not to change, it means we never get converted over, and new stuff gets made in inches, it just kicks the problem forwards. Very often those who say Inch is fine, or claim it's how they work are in a bubble. They really want to think what they say is the case, but don't realize how much down stream or parallel to them work in metric and the problems it causes. I'm sure many of us have worked in businesses where things get converted back and forth multiple times as it works thru the processes of the company/suppliers/customer/etc as everyone has their own idea how things are done, or what is preferred. Folks tend to make decisions/statements based on what they like/what works for them, not the bigger picture. Which gets back to the US being less than 5% of the world, with major industries being full metric, and the shift to metric continuing (there is not growth in Inch usage). This is where personal preferences doesn't matter. It's about matching up to the world and everything around.
On drain pipes, far as I know, Europe uses the same sizes at N.A. they label them different, but dimensions are the same. Where things become issues are supply. Their supply lines are mm, where we have the CTS sizes (nominal plus 1/8"). When PEX came, it should have been left as is, there would have been a natural conversion to metric sizes. Then everything that is made globally for plumbing/heating/cooling could be used directly. As is, you have a very real conversion problem. Money was spent to re-invent pex and the related bits to match up to the ODs of legacy copper systems, so now anything supply plumbing brought to the US market has to be re-designed/re-tooled for the US market, which cost a lot of money, which is spread across a much smaller market (5% vs 95% planet). Verses having just made some metric to inch adapters decades ago to interface old to new.