The UK Metrication Association in 2006 estimated the cost of converting road signs to be £80 million. Only the road signs.
Now consider UK and the US as a whole- all of the cost to the government, businesses, non-profits, etc.
I assure you that the UK spends that much or more on signs that were damaged by weather, were rendered incorrect by road changes, or needed updates due to new signage standards.
When the US automotive industry looked at metric as a big push was being launched they realized that metic was cheaper and switched. Fewer bolt sizes, fewer tools required, simpler designs, easier math for machining. There was literally no reason to stick to the old system.
Stubborn is not harsh. Stubborn is the truth.
There's nothing sadder than watching some American mechanics struggle to add together things like 7/8, 3/16, 1/2 and 17/32 based on different bits of material they need to assemble together, except maybe watching the team on Mythbusters flail as they insist on using ridiculous units like inch-pounds and feet per second to try and do basic physics. How many feet per second is 66 miles per hour? What kind of inch-pound torque on a six pound weight at the end of a yard long arm through an eighty degree arc generates that kind of speed?
The big perk of Metric is all the units are normalized and mostly interchangeable. The big nuisance of other systems is the units are all completely arbitrary and require conversion factors.
Get with the program already. Metric might feel annoying, but that's only because change is difficult. It's natural for anyone living in a country that's flipped.
The issue of cost is not unique to America.
1. The customers use inches.
2. The expensive machines we use are configured in inches.
3. The code we use uses inches.