It's like translating grandma's units of measurements from her old recipes.
"A smidge of this, three sprinkles of that, and a can full of something that does not come in a can" (The can was her 'measuring can' that was some kind of weird size that doesn't exist anymore)
Edit: After she passed, we threw the can away because we didn't realize it was the can and load-bearing to most of her recipes. After some best guesses + trial and error, we concluded the can was approximately 7 ounces / just under a cup.
Grandma was a constant ball buster and, as my aunts and uncles weighed her prep carefully, said "why? You're still gonna mess up the cooking. Stick to baking."
When they said "7 ounces / just under a cup." that's not a particular physical cup. A "cup" is an exact measure in the US, it's 8 fluid ounces, which is 236.5882365 milliliters precisely.
Even if the US does go metric, it will take a lot longer than 50 years for people to not know how big a cup is, all measuring tools in every kitchen are marked for them and the other common units like tablespoons and teaspoons*, and virtually every recipe uses them.