Do you honestly believe that we're thinking about temperatures in Fahrenheit and then just converting to Celsius when we write them down?

Why on earth would I think that? I made the comparison to other units of measurement to demonstrate why smaller units are useful in some cases. There are cases where its not useful, but there are also many cases where the advantages of Celsius aren't useful. Neither is inherently better, the correct one to use is the one you know better or the one that fits the use case better.