I can see someone going into a lumber yard and saying...I need a 2x4 and getting an answer of “We only carry Metric lumber anymore!”
Nothing will fit his house!
Shucks! I’ve never figured out Rods or Knots, and an electrician will play hell trying to figure out the electrical load in Joules.
Joules are total energy. To figure out a power load those electricians will have to understand the tricky metric power unit of .... watts. A joule is one watt-second and a kilowatt hour is 3.6 million joules.
One job I had did everything in metric because we had so many foreign customers. Any measurement in inches had to be converted from internal measurements in millimeters. It is far easier to get a 6mm bolt here that a 1/4" bolt in the field in a factory in Whogiveadamistan.