I have had too many students tell me an electron that passes through a weak field will end up with a velocity of something times 10 to the 9th power m/sec....faster than the speed of light. So even with ease of conversion between units, it is very easy to screw up if your brain is not turned on.
Meanwhile, outside the classroom or engineering firm, it is very unusual to have to convert from miles to inches for anything practical, or for that matter from km to mm.
The point about exports is also incorrect. The US and Canada are each others' largest trading partners, and I see no serious issues due to the fact that one country uses metric and the other doesn't.
Finally, if a country converts, there is still the matter of legacy measurements, especially in areas like real estate, so the population needs to learn both metric, US units, plus the conversion factors between the two. So the argument that metric is simpler in this case won't hold.