English and Germanic r.

There might be an easy answer why the English language use a
retroflex approximant r.

I think it is the result of the need to have audible differences
between phonems.

English has a voised dental spirant as in "they". This spirant has
an accoustic effect very similar to the apical trill.

The use of a retroflex approximant r, makes the difference between
the two phonems clearer.

Since the dental spirant is of proto-germanic origine, the retroflex
r may be an old phenomenon.