"The point is that you quickly come up with short forms of long words, in whatever language, so that in itself is a poor argument against switching to metric."

Yes, that is true. In Swedish and German "kilo/Kilo" is used for "kilogram", but "kilometer" is always in the full form in Swedish. In Korea they mostly use "kilo" for "kilometer", but they use it for "kilogram" as well! I guess context helps al lot.

"in casual speech, people in metric countries say "k" for kilometres"

We do???

I've lived most of my life in metric Sweden, and for years in Germany and in Korea (both metric). I've never ever heard anything like "k" for "kilometers", except for the abbreviation "kmh" - "ka em ha" - that the Germans often use in speech, but I've never heard any German use "5 k" for "5 Kilometer".