I haven't seen it mentioned, so I'll say it here. My work van has about 454 kg of tools in it. That a lot of crap that I would have to duplicate in metric if everything started showing up with metric bolts, nuts, and screws. I'll tell you right now that my van doesn't have space for that increased tool load. I'll also say that I regularly see equipment that is better than 40 years old. It's the back end that really makes it hard to switch to metric. Ask any auto mechanic. Every GM car out there has both metric and SAE bolts in it. It's a major pain in the butt to figure out what you are working on.
Rotating assemblies is where I would have the largest problem adapting. A standard 56 frame motor has a 5/8th inch shaft. That shaft mates to a fan section or pully where thousands of an inch count. So now if I crater a motor and the only thing I buy is a standard metric size, that means I get to replace the fan wheel as well. Now, can I find a fan wheel that has a metric shaft hole that still matches the dimension of the cage it rides in? Am I going to have to replace that fan cage as well? Is the new fan cage with slightly different dimensions going to move the correct amount of air? Am I going to have to adjust the TXV to account for the increased/decreased airflow?
This stuff snowballs really quick. Pretty soon it's a lot like having a hard drive crash. If you are doing the work yourself, it's cheaper to buy a hard drive and install it. If you are paying a tech to put it all back together for you, lots of times it's cheaper to buy a new machine.