It's expensive, the benefits are minor, and our system is fine (it could be better of course: a switch to nautical miles would be nice, and making a gallon 256 cubic inches would be great). I've never seen a compelling reason to convert — and neither have Americans in general.
> Every other country faced the same expense and learning curve.
If every other country jumped off of a bridge, would you? The French system really isn't as good as it's cracked up to be.
French units optimise for abstract unit conversion (e.g. inches to feet or millimetres to kilometres); standard units optimise for concrete manipulation (e.g. dividing a gallon into cups, or a litre into decilitres). The thing is, unit conversion really isn't that common compared to manipulation (after all, what do units of measure exist for if not to manipulate objects?).
