I'm a fairly experienced machinist, though it's not my occupation. At the present time, machinists have the least problem with metric. The unit conversions are built into all of the machines and measuring tools. You press the inch / mm button. The ruler has inches on one side and mm on the other.

Everything is standardized on IEEE floating point. ;-)

It's a headache to maintain collections of parts and tools such as taps and dies for both standards.

The biggest shift is simply the obsolescence of old stuff, and emergence of new stuff. And industries have adopted the practice of reducing the overall variety of parts needed. I work in the development of industrial measurement equipment, and where a design might once have had 30 different sizes of fasteners, now it's 5, all metric. Designs rarely need nuts and spacers any more. Washers are integrated into the screws. No more "philips" or flat head screws. And so forth.

> Designs rarely need nuts and spacers any more.

What is the modern approach where nuts/bolts and spacers would have been used?

> No more "philips" or flat head screws.

Torx? Rivets? Something else?