I read the edits on your original comment. I think you're missing the point that the metric system at least scales in a reasonable and expected way, which is not the case in the imperial system of measurement.
And to your point about averaging a human arm and using that as a measurement standard, no, it's not necessarily better. You still need an exact standard that you have to measure against for any kind of precision. Making it similar to the size of a human body part doesn't matter unless you're estimating, which isn't how anything is built anymore.
There are reasons the old standards were abandoned.