Everyone knows the SI system, Mike. We just don't use it much except in international dealings.
Inside a country, people use whatver customary units they want to use.
When we order off the shelf machines from Japan or Europe, we take the metric hardware. If we contract for mahines for domestic use, we simply speficfy SAE hardware be used and holes readjusted for correct size. If they don't want that business, they are free to say no. To date, we have had ZERO trouble when we specify SAE hardware be used and holes chaged to accommodate same.
If someone has the only game in town, you accept their hardware and buy some spares. If there is competition out there and they want the business, then they accomodate, not the people ordering.
When the US government accepted the metric standard, they didn't ask the people, they accepted it for government reasons and told us what they did. So far, almost nobody has taken it up and if they force that, we can always vote their butts out of office. Don't think it wouldn't happen.
But when I see units in SI, it is not difficult to handle in any case, so I am comfortable in several systems including SI, CGS, MKS, English, and a few others. There are a lot of people that way who have no trouble whatever the units.
The onlt real trouble I had was buying an Italian machine where the air pressure was specified in nl. My Italian engineer friend kept saying "norma-litres" and we couldn't ever figure out what he was meaning. Turned out after some investigation he meant "normal Litres," but he hever wrote it out and never DID convert it to psi for us, so we rejected the $150,000 USD machine, returned it at his expense to Italy, and bought a US product.
Later on, I found normal litres as a unit and would have no trouble converting to to psi today. The conversion was necessary since our entire factory ran on psi air pressure, not nl.
I'd suggest if someone is having trouble with a US company converting to SI. then someone somewhere is mis-pronouncing the unit. The solution is simple, SPELL the damed unit out and everyone knows what you mean. Pressure is force per unit area and we would have had no trouble with SI pressure ... and nl is not a standard SI unit or an English unit.
The only one we just grin and take today is Whitworth tools and fasteners because all the Merlins that exist have already been produced. So if you want to fly a Merlin, you get Whitworth tools and fasteners or find another warbird that takes another engine. Since we fly three Merlin-powered aircraft and race another one at Reno, we don't have any trouble with it. But every time we have to replace something on our A6M5 Zero, we convert it to SAE. So today we have a Zero that is somewhere around 75 - 80% SAE except for the engine, which is all metric. Makes working on it a challenge, but the plane is a beauty.
Oh yeah, all our rivets are standard AN rivet sizes, no matter what they originally were. If we ever replace one, we replace it with an AN 426 or AN 470 rivet, and when we replace sheet metal, it is with 2024-T3 in inch sizes. Mostly .020", .025", .032", .040". .063" and the like. You won't find any mm size Aluminum at Chino unless we are throwing it away and replacing it wqith now sheeting.
It is expensive enough to stock all the WWII normal sizes much less the modern stuff that our planes never used anyway. Our newest jet is Korean War vintage, and it still used all SAE and English measurements.
For our Hispano Ha.1112, we are finding metric bearings because the bearing races are all metric and we'd have to machine it out. It is easier to get metric bearings than to do all that machining.
So my understanding is simple ... the buyer controls the units used, not the seller. If the product is unavailable except in units the buyer refuses to accept, the buyer can convert it at some expense, find another product, or do without. One of the options will be taken as there are no other paths to travel.
I can guarantee that if the USA comes up with a military weapon or technology that can defeat some particular threat technology of interest and it comes only in SAE, then Europe/Japan will take it in SAE, forgo the weapon / technology, or pay the cost to convert to to metric. There aren't any other options.