the length of the path traveled by light in a vacuum in 1/299792458 of a second

it's also 1/1000000th of the distance from the pole to the equator, you can do this math yourself with basic tools to "everyday" acccuracy, just like a kilogram being a litre of water (with a much fancier process to get it physicist-calibrated). not equally arbitrary. a hand or a foot is a distance that has never existed in a replicable form beyond a single body, the dimensions and physical characteristics of observable, stable natural phenomena are the same for everybody, everywhere.

but lets get back to your absurd sequence of events "someone eventually said "lets all agree to use the King's foot as the standard" that someone being the King, the "agreement" was the coercive power of the state to enforce that ruling. what if the king's foot was unusually short/long? you had to use it all the same. this fantasy you've constructed about the humble ole' conventional units doesn't agree with the basics of how states work, no matter how euphemistically you present "the guy who draws and quarters people for not bowing low enough decided his foot was a standard of measurement" there was absolutely nothing democratic or 'bottom-up' about it. you want to hear about democratic processes? how about the hundred republics that have gone metric without the threat of force? the peoples republics, the soviet union, the dozens of countries that became independent from britain and adopted it?