That said I think meters and Kelvins are doing their job just fine even for use with rocket science. I'm glad I learned the SI units at school, they reduced the number of constants a lot in physical formulas.
i don't think it makes sense to describe miles or meters or stoney lengths as 'wrong' or 'right'; you can express the fitzgerald contraction or orbital speed or whatever equally well with any of the three, but one of them simplifies the fitzgerald contraction somewhat
agreeing on distances with a faraway colony is actually a considerably easier problem than the kilogram; if we transmit them a radio or laser message, they can measure its length to within parts per billion, and we can do that with 01950s technology, while kibble didn't invent the kibble balance (previously known as the watt balance) until the 01970s
(you do have to worry about redshift: one part per billion of redshift is 300 millimeters per second, so you have to know the relative velocity of alpha centauri to within meters per second to correct for it—and, while that's also the 01950s technology of measuring spectral line frequencies to that precision, that same technology is what allows you to do without the radio message entirely)
transmitting or storing the number "9192631770/299792458" can be done with significantly higher exactitude, of course, and can be done over twenty millennia more easily than transmitting a radio message can
Uhm, a kilometer is supposed to be 1/40000-th of the Earth's meridional (i.e. from South to North poles) circumference. The modern value is 40008km, so the official meter is juuuust about 0.02% shorter than it should be.
whether 0.02% sounds ridiculously good or ridiculously bad depends on your frame of reference
https://en.wikipedia.org/wiki/History_of_the_metre#History_o... says that the original mètre des archives in 01799 was machined to within 50μm, which is 0.005%, four times smaller than the error in delambre and méchain's computation. so even at the time that was a pretty large error. since then the measurement uncertainty of the meter has improved by five more orders of magnitude, to about 0.1 part per billion. 0.02% is 200 parts per million, or 200000 parts per billion, which is a lot more than 0.1
i ask you, what the fuck kind of number is 9192631770? is that a hexadecimal encoding of linus torvalds's first child's birthdate? no, it's just a random fucking number derived from the sumerian base 60 numbering system and the average rotational speed of the earth in the 20th century. the kelvin has a similarly filthy history; it's when the thermal energy changes by 1.380649e-23 joules, because that makes the triple point of water come out to 273.16 kelvins. so now we have to preserve those numbers for all eternity like they're the fucking holy writ of the priestesses at delphi in order to interpret scientific papers from the 19th and 20th century. (and don't get me started on the motherfucking calendar. jesus jumping blue christ.)
you will be pleased to learn about https://en.wikipedia.org/wiki/Natural_units (which i used in https://dercuano.github.io/notes/2017-sap-allocation.html because i'm uncreative) and https://en.wikipedia.org/wiki/Duodecimal#Systematic_Dozenal_...