The point is that the numbers you are using are *inherently* base 10.
When you say
2394.493,
it literally means
2*1000 + 3*100 + 9*10 + 4*1 + 4/10 + 9/100 + 3/1000.
Each "decimal place" (note "deci" = "ten") denotes an additional power of 10. This is why multiplying or dividing by 10 is trivial.
2394.493/10 = (2*1000 + 3*100 + 9*10 + 4*1 + 4/10 + 9/100 + 3/1000)/10 = 2*100 + 3*10 + 9*1 + 4/10 + 4/100 + 9/1000 + 3/10000 = 239.4493.
You can simply move the decimal place. There is no actual computation, because the notation itself incorporates the arithmetic.
Our numbers imbue tenness.
You could set up a number system to be of a different base, like 8 instead of 10. That would be fine. Then when you say
423 you would mean [4*8^2 + 2*8+ 2 = 274] in base ten. And then I would totally agree that it doesn't make sense to use dm, cm, mm, instead we should have made up words for 1/8 m, 1/64 m, 1/512 m.
But our numbers are naturally aligned with ten, so our units should be as well.
Am I misunderstanding your argument then? I thought you didn't like Imperial Units because it wasn't based on Base 10, but you are clearly showing that:
12.00 miles uses a base 10 numbering system, just like 19.3121 kilometers. In both cases, you can just move the decimal point to get 1/10 or 10x the number. In the shown example, it turns out that the Imperial Unit happens to be even easier to divide (by numbers of than 10) than the Metric Unit. That's just because of the numbers I chose, though.
So, as you just proved, if both set of measurements use Base 10 numbering systems, you are actually arguing about the words we use to denote specific amounts of those numbers, which again (when ease of use/math is removed) is just an aesthetic argument, which I'm making in favor of Imperial.
1 kilometer = 10 decimeter = 100 centimeters = 1000 millimeters = 10000 µm (whatever that is)
1 mile = 8 furlongs = 1760 yards = 5280 feet = 63360 inches
That's a fair comparison, when it comes to length? Now take out the numbers, because we've now agreed (as you've taught me) that both systems use the same number system.
kilometer = decimeter = centimeters = millimeters = µm
mile = furlongs = yards = feet = inches
This is the discussion we're having. I can see why some people might prefer the top line (those who like repeating letters, rhyming patterns, etc.), and I can see why some people might prefer the bottom line.
In the end, though, there is no definitive argument one way of the other than can convince either party. It's completely subjective.
(Note: it took me a million clicks on wikipedia to find the next step down from millimeter; why is it not all on one page?)
Edit: fixed my math