I don't really see more of a problem with different systems of units, than different fastener types, we have traditionally used robertson or slot, then phillips now there is star bit.
I have adapted many pieces of equipment (threaded hubs) from/to metric threads to NPT.
I have installed metric cable tray in the USA and even metric cables in said metric cable tray. I have used metric cable glands... Not sure why its big deal to not have any metric wire in the NEC. NEC is not a design manual, it should have a table for metric wire (EDIT that is if doesn't I am not that up on the code LOL)
I have adapted many pieces of equipment (threaded hubs) from/to metric threads to NPT.
I have installed metric cable tray in the USA and even metric cables in said metric cable tray. I have used metric cable glands... Not sure why its big deal to not have any metric wire in the NEC. NEC is not a design manual, it should have a table for metric wire (EDIT that is if doesn't I am not that up on the code LOL)