I kind of get it, it's like language immersion. How do you easily describe anything besides the freezing point and boiling point of water in an objective way? The rest, you can point to and say "this weighs a kilo" ot "this holds a liter." And if you don't force people to use it, they'll simply refuse. And we all carry handy unit conversion tools with us wherever we go these days, so if they don't want to learn, they can easily translate it themselves.