All of my science classes in the rural US from middle school onward were taught entirely in metric, and it had been that way for decades prior. Obviously I'm an adult now, and metric still isn't universal.
I did the math on a comment elsewhere. Just google how many miles of interstate there are, and quadruple it, thats not even approaching how many signs you'd need for just the interstate. Signs run $25-ish, plus installation, which can cost >$1000. At the very least the km markers will need to be installed.