I live in an area of Europe that used SI units, but did use customary units before. I think that part is conveniently forgotten in discussions, it’s not like America is exceptional in having some heritage units. Other parts of the world just managed to migrate.

Anyway what I say going to say is there was a migration Path here which went along the lines of changing from a base-12 conversion to a base-10 conversion. So there suddenly was a customary unit and a transitional unit. After that metric base units were introduced