A common misconception is that pounds and kilograms, or nautical miles and kilometers are similar units. They are not actually measuring the same thing. The pound is a unit of force not mass, kilogram is a unit of mass not force.(newtons are the SI unit of force, and slugs are the "imperial" unit of mass) kilometers and statute miles are units of fixed distance and do not cleanly divide into the Earth-geoid's circumference, nautical miles are a fixed integer fraction of the circumference. What matters most to an aircraft(which tend to have minimal acceleration once air born) is not the mass but the lifting force, drag force, and motive force(thrust).
As for nuts and bolts etc, all that matters is that an industry standard, "size B" just needs to be consistent between makers and batches, the units used to measure the tolerances of a size B make no difference. The set of standards to be used is purely one of economics, this is why all plumbing in buildings around the world is based on inches, it is a standard so all spare parts fit and mass production makes it low cost. That is what counts, that you call a pipe one inch, 26mm, or size 37G doesn't change the fact that it has a known diameter tolerance thread pitch, thread form, and is compatible between manufacturers.
When it comes to machining many machines have, for example, set gear ratios for either metric or SAE forms (maybe carriage movement per rotation of a lathe) and these are very costly machines that last for many decades so just switching may not be cost effective, especially if all the machines need to switch at the same time. American auto makers attempted to mix metric and inch withing single vehicles during the '70s and '80s to ease the transition costs and it was a nightmare for maintenance mechanics.(really one of the several reasons for the downfall of Detroit)
The issue is eg a 12mm screw is very close to a 1/2" screw, close enough for general strength but not close enough in thread shape to interchange but too close in size to notice by eye. Also you need two full sets of tools to cover the same size/strength range.