Most things have to fit together with something else so they have to change over at near the same time or things go south. For example, in housing the length of studs would have to change to accommodate the change from 4x8' sheets of plywood and sheet-rock to 1200x1800 mm. Coming up 3/4" different just wouldn't work or would cost a lot of money cutting off the excess 3/4" (~19 mm) of material, whichever side the excess is. There are also engineering problems where old designs won't easily convert. For machine designs some bolts have to fit within 1/8" (~3 mm) or less of tolerance. A 1" long bolt is almost 5 mm shorter than a 30 cm bolt and a little over 5 mm longer. Anything with these issues would not only require retooling but redesigning as well.