try measuring the distance from the pole to the equator in the case of civilisational collapse

well, technically you can, but it would take you several days and a very long walk between two poles on a sunny day for a likely inaccurate and unreliable result. Eratosthenes got 2% error, but how do you know your measurement will be that precise? What will you do if your measurements are challenged? More importantly... how will you measure without knowing what a meter is?

200 years from now in the apocalypse someone could say the earth is the radius of the moon, or that the earth is flat- and so your measurements can't be trusted, nobody would be able to prove otherwise.

but one foot is trivial to measure, even if it is somewhat inconsistent. If you need 'more', you define it in terms of practical things you can compare it to without much effort or thought- such as acres being measured in tillable land per day.

The rest can be standardised later.

Culturally, the US customary units have an implied inconsistency. When someone says 'walk 10 feet', you can in your head estimate they want you to make 4-5 steps and you know it's approximate. When someone says, 'walk 10 meters' then suddenly the required consistency is context dependent as 10 meters is a lot harder to visualize. I suppose you could think of it as 'the length of 5 tall people', but most people just don't think in those terms. Eventually, as civilisation rebuilds people will need to come to an exact agreement on what a feet etc is. At which point the metric system begins to make more sense. Until that time, however, the customary units suffice as is.

you are right about temperature though, Celsius is a lot more practical. Farenheit is defined similarly but is based on some random estimate of human body temperature.