Officially, the United States and some other countries have defined non-metric units in terms of metric units for over 50 years.
Unofficially: The metric system has some units that you can directly perceive and understand: a meter is a little bigger than a yard, a liter is a little bigger than a quart, a kilogram is a little bigger than two pounds. ("Related to some portion of the body" is, AFAIK, specific to feet.) I'd argue that the big mental block is that you can "see" and are accustomed to multiple base points for the same type of unit (e.g. inch, foot, yard). Also, you can sort of "see" miles as its own base point, roughly equivalent to "minutes driving at highway speed" (at least in the absence of congested traffic).