I need you to understand that those phrases are still used all the time in English speaking countries that have adopted the metric system.

Changing your scientific measuring system doesn't necessitate a change to your colloquialisms; commonly used phrases don't change.

In case you are not joking... in metric countries we still say the same sentences you say, with miles and inches. That doesn't mean we use the olde units to actually measure something...

Well, then you are welcome for us keeping the more poetic units alive and meaningful.

Sorry, but no. They are just our old units that stuck in everyday language. They are probably not even closer to yours than yours are to antique Egypt ones, given that they were not internationally standardized and merely referenced the same body parts.

And we have expressions including other old units that you don't have (lieu, arpent, toise, etc.).

Not really --- in some languages these sayings actually are already in metric units. The SI system is by now over hundred years old, so just by waiting probably will upgrade unit system also there.

In Russian you still have people using colloquialisms and idioms with units that were put out of use almost a century ago, yet everyone here gets the meaning (or at least the intent, which is what matters in the contexts they are used in) since they have stayed in the literature written centuries ago and of course nobody went out of their way to fix measurements in works of fiction.

Heck, most people using them don't even have a rough idea of how much they originally meant, and if you were to press them to give you a number for the sake of an experiment, they might be 2 orders of magnitude off.

https://en.wikipedia.org/Obsolete_Russian_units_of_measureme...

Yes, that's true. Another argument my American coworker had put forth: "American rock-n-roll needed Imperial units... or the lyrics would not work".

I think there's a serious point to what you're saying. I was bought up in the UK so I have metres for short distances and miles for long distances. If you say to me 5 miles I instantly know what you mean. If you say 8km, I have an academic understanding but not really an intuitive feel for what you mean (other than 8km=5mi, feel 5mi).

That's just familiarity though. People in countries that primarily use kilometers have the same intuitions about them.

I doubt that in practice an average person would be able to (reasonably) accurately differentiate between 4 miles and 5 miles or 6km and 8km without external measurement tools in an unfamiliar environment.

The heuristics at play that the human brain would use is likely: "5 miles is way more than I'd normally want to walk on my feet, since the trip would take me about 2 hours."

As our metric British friends say at the end of a long day: "Ah, I can't wait to get down to the pub and have my hard-earned 568.261 ml!"