Am not sure I completely agree with you. Some of the archaic units are still used because USA is so dominant in both technology and advertisement. People living in other parts of the world know exactly how much inch is or gallon. Just like most of the world knows English, even though it’s not the easiest language to learn. Simply GB was too big and influential and they left their mark.

As far as Mm is concerned, why would you do that? If you were making a small panorama or model you wouldn’t use 0.00001km, would you? You pick units that are most convenient for the purpose. Kilometers are used for cars and things related to traveling because fuel economy is expressed in such unit, car’s own computer measures distance in same unit, speed is measured in same unit. Why would anyone use anything else? 0km until 1000km is perfectly intuitive scale which doesn’t get crossed too much. For the very same reason that’s why we use square meters to measure surface of a home, because they never go into square kilometers. And square milimeters is pointless.

It’s all about practicality. SI system is great because it allows users to use same unit in different scale and have it be intuitive and easy to convert. Also when it comes to astronomy, there are many units you skipped there. LY is too big and rarely used unless it’s to describe distances to other galaxies and size of those. There are AU as some one else mentioned. Earth size, etc. But rest assured when scientists are trying to calculate something, they still revert to good old reliable SI system.