I don't argue the points you make. As I said before, I find the metric system easier to convert within its own system than customary units. It IS taught in U. S. schools, in EVERY school (so far as I know) at multiple points in a student's education. If they used it in every day life, they would retain it, and it would be natural just like using a foreign lannguage. But, like a foreign language, when they don't use it, it becomes just a bit of theory that gets filed away and becomes dusty/rusty.
If the U. S. government said you WILL convert, in 60-70 years the U. S. would be a metric country, once the generations that grew up with customary units had passed on. (Much the same was the case with Japan, which took 78 years to fully integrate, and the crucible of WWII I suspect helped the population to adapt more quickly than our country might.)
But the likelihood that the U. S. government will pass laws changing our system of measurement is very small. In international business, we DO use metrics. But in day to day life, the will of the people just wouldn't tolerate it. For many, tradition is worth a bit of reduced efficiency.
And the inefficiency is not of a degree that some might imagine. In the tire factory where I worked for a little over a year, tire parts were measured in tenths of an inch. When set for overseas companies, we still used tenths of an inch. The truth is that the tolerance in building a tire is enough that we could use a tenth of an inch (= 2.54 ml) for the internal parts (belts, ply, chafer, etc) and produce a tire that converted, after curing, to metric units and international tolerance with no problems or issues of consistency.
As for fluid ounces having different values, a US fluid ounce measures out to 28.41 ml, a UK fluid ounce to 29.57 ml. In a chemistry lab, this difference might make a miniscule difference if compounded in to many many gallons. But not nearly as likely as you might think. And when measuring fuel for testing an alky stove in a garage, 1.16 ml is the amount that might be accidently left on the threads of the bottle. It might be noticeable with a graduated cyclinder, but not in the real world. In the backcountry, with the measuring devices used (such as a nalgene or lines scratched into a mug or bottle), the difference for practical purposes between a US fluid ounce and UK fluid ounce is nil.
Americans use the metric system when they have to. But in daily life, we simply don't have to.
My principle point in this discussion is that I strongly disagree that we should do away with customary units ON THIS SITE. I support the idea of dual listing of measurements, both American and SI. As a sign of my sincerity, I'll gladly include a metric equivalent with US units whenever I give a measurement in my postings. (And you may have noticed that in my couple of hundreds posts that I already respond to international users by giving metric units out of courtesy.) I just hope this discussion raises an awareness that among many Americans (though not all of course - Americans aren't completely united on any thing), this is a very serious cultural issue. Measurements should be listed in both systems.