Ah, not quite - I'm operating on the assumption that the people working for thousands of years arbitrarily pulled them from thin air. Because, you know... That's literally how all definitions of measurement work. Try the cubit, somewhere between a foot and a half and 4 feet - what was so useful to people about those distances? At best ones like the foot and the chi came from using your forearm as a standard length, but once they're standardised the choice of whose forearm to use is completely arbitrary. Russian units were based on different ways of stretching your body parts - stuff like a hand span or arm span. Choosing what to use as a measurement is completely arbitrary, which then is standardised over time by people using the same thing as a measurement and it eventually getting defined as a single distance. The kings arbitrary choices were just building on the arbitrary choices of the past. There wasn't anything less arbitrary about the choice because lots of people used it.

Yes, they discovered the law as they understood it was mistaken, and so found more accurate ways to define it, which is why we're now using a fraction of a light second. If we find more precise definitions in future we'll define it by them.

Lastly, they already did that. The imperial and us customary measurement systems are both defined by metric distances now, specifically because metric distances, equally arbitrarily chosen as they are, shouldn't change. And if they do, we'll find something that doesn't.