"Going Metric" really has nothing to do with measuring in Centimeters and Celsius and never did. It really has to do with retooling industry and parts to new standard sizes. The problem is it is very costly to do so. Think about the metric and common wrenches you have, is used to be you had one set of tools and best of all because there were not really that many commonly used sizes, and the differences were visually apparent, you could just reach for the right wrench by looking at it. Right now you might be thinking "ah but of only the U.S. would just use the parts the rest of the world uses, things would be fine.". First, remember that goes both ways as metric nuts are in no way better than common ones. Second, well, frankly most industries have fairly specialized tools, in other words you care more about those immediately around you, the sizes of available nuts have nothing to with board lengths.
But, we are entering a new custom manufacturing world. We may soon be manufacturing boards AND nuts to the specific needs of the product rather than standard sizes. When that happens, well, you can measure in inches, meters or cubits and the computer will be able to convert and manufacture just fine.