0 degrees Fahrenheit is really cold, about the coldest that one can stand by simply bundling up, without having to resort to special clothing. 100 degrees Fahrenheit is pretty hot, about the hottest that one can stand without having to take special precautions with hydration and attire. By contrast, -18 degrees Celsius and 37 degrees Celsius aren't terribly intuitive.
Perhaps that is simply because you are not use to Celsius?
0F was suppose to be the coldest Fahrenheit could get. He used brine of ammonium chloride and marked this as "zero". I suppose based on your statement that -18C and 37C are not intuitive means you are very familiar with ammonium chloride brines?
Next, 32F is the temperature with a mix of 1:1 ice and water and based on this he was able to determined water boils at 212F.
0C is really easy to understand, it is the temperature at which water freezes. not ammonium chloride, not water at +3ATM, plain water.
In Celcius, water boils at 100C, again plain water, boiling and again something that is common and easy to understand.
So in short, ammonium chloride's freezing point = Intuitive
Water's freezing point not intuitive?
"SI also lacks a good equivalent to the Foot. Decimeters are only about 4" long, and meters are over 3' long, so nothing in between. "
Why does it need an "equivalent" when it is not the same system? Instead of saying "one foot" you can simply say "30CM"
You are trying to take the old imperial system and look for a metric equiv but since the systems were constructed differently no such equivalence exists.
You obviously were raised on the Imperial system and you understand this but to people raised on metric the imperial system is equally odd.
SI reminds me of hyperinflated currencies, where the units don't align well with real-world uses. I like the idea of base-ten conversion given our current numbering system, but the scales are off.
Not sure how SI/SAE relate to hyperinflation, suspect you are misusing the analogy.