> Any manufacturing component which cannot easily be adjusted to a slightly different measurement is poorly designed, in my opinion.

Imagine all of the parts in a car engine. Everything fits together perfectly. Engine mounts line up in the right places. It all has to be very precise.

Now imagine you take all those parts and round them off a bit. Nothing much, just a mm here and there. If you try to put the engine together with these parts, it's not going to work at all.

Also consider that you can't just simply convert the units. Take a 5/16" socket wrench for example. Nobody makes a 7.9375mm socket wrench.