and which one is a quaint and charming relic of a bygone age.

The US is still a global hegemon... We still all have to deal with your measurement system regardless of whether we actually use it ourselves, and it is not quaint to us. We also have a few market units still in use, but most units fell out of use without any need of force because there are real practical advantages to metric units. Many of these are rural situations, but urban workers do not feel the same benefits from these units. Others like grocery weights have stuck around and been rounded to close metric values for easy conversion.