E.g., after WWII, when the US Springfield Armory created and manufactured the M14 rifle for the NATO 7.62x51 round, it could easily ensure all cartridge and NATO STANAG were in both US Customary and ISO Metric down to at least 5 significant digits on machines for any machine made since virtually the start of the 20th century. This is why US manufactured M14 rifles have no problem using Metric magazines, interchanging with the Belgian FN FAL, and other NATO focus with magazines. In fact, even to this day for the M14 and other rifles that have been re-issued, the US calls the "Metric magazines" when differentiating from the British SLR.
The British license built the FN FAL as the L1A1 SLR, but still used UK Imperial units, which did not translate remotely as well, resulting in "Imperial" magazines that were not STANAG compliant. This became readily apparent during the Malvinas/Faklands crisis when the L1A1 and FAL could not use the same magazines, or not without issues.
The US officially went metric in the mid '70s, and all American children have
only been taught Metric over the last 40 years. All US goods have had to be dual-labeled since then, and new labels tend to be metric default (e.g., nutritional content). However, the sheer existing US manufacturing equipment, land plats and surveys and other things tend to be US Customary, and remain because they have Metric exactness up to at least 5 significant digits (basically, it's as accurate as measured, just as if Metric too) had assured -- again --
US manufacturing never had the problem that UK manufacturing did.
Newer technologies, like Integrated Circuits in Electrical Engineering, has long move to Metric, while Mechanical and Civil tend to use dual to ensure there are no issues in the future at higher precision.
E.g., electronics initially started using mils (based on the inch) as that was how US electronics long worked pre-integrated circuit and even microelectronics. But when the US started to re-tool for surface mount in the late '70s, it switched to Metric as the older manufacturing equipment had to be replaced.
Other than very ultra-precise industries with long histories that pre-date the Metric change -- like SAE (automotive) and SAMMI (firearms) -- you will see Metric being the default tooling. Metric is still required in all standards now, but those older industries and standards will default to US Customary for historical accuracy. But this tends not to be an issue, because the US defines things differently in some cases.
E.g., in gunsmithing, the US v. EU differs on whether bore or bullet is used, as well as neck angle (cartridge), which means it works out very well for both systems (one favors each). But the US military uses NATO, which is ISO Metric ... always. US slang is used at times (50-cal), but is not the standard at all in anything (other than pre-dating equipment -- e.g., M2 is over 100 years old).
Otherwise, only the US media still only uses US Customary, because they are the least scientific and engineering embedded. They are also the most ignorance of science and technology. Heck, they are even pushing various theories on the Moon Landings and 9/11, which the NIST -- ironically -- has gone to great lengths to product reports on that anyone with a high school level of 18th century physics should be able to follow. But, alas ...
The road sign thing is the same issue as the UK ... people reading the higher kph and pleading ignorance. So the courts continue to favor mph on the roads, although there is a general move to use meters instead of yards (and yards, instead of feet, was a tactical move in the mid '70s). As far as FAA, I agree, it's time to drop feet (that was just wrong), although nautical miles (neither US nor UK miles) is still going to be used for non-computer based instant-calculations based on degrees ... and will always be taught as a "backup navigation" measure.
Disclaimer: I am an degreed EE that spent some time in aerospace (mechnical) and semiconductor (electrical, my focus), but grew up in a land surveying (civil) household and basically have equivalent experience to be licensed in that field as well.
