One of the Mars landers famously failed due to unit conversion errors from metric to standard.
That said I wish the US would bite the bullet and make the switch. Mandating dual labeling on everything would be a great start. Then in 20 years we could narrow it back down to one.
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.