It's really not that bad. Canada did it in the 1970s and it's still around, and we didn't revert to the stone age or anything.

SI units are already in wide use in the US, for instance for electricity (Amperes, Watts, Volts). For the most part, international trade has pushed US industries to adopt metric anyway: metric fasteners are used all over, in cars for instance. Also, countries that have switched still use inch-pattern stuff; in Canada we still use NPT threads and imperial size pipes with no ill effects; even in Europe they still use some imperial-threaded stuff.