Why are prescription frames so rarely canted down at all?  Your normal
gaze tends to be somewhat downward.  Your skull geometry blocks upward
angles and accomodates downward ones.  Even when looking ahead, you are
scanning downward such as to avoid tripping over things.

Any reason why I shouldn't bend temple arms near the hinge to cant the
frame down a bit, like so many dark glasses do anyway?  Even with this
I would be undercorrecting; it seems most frames are excruciatingly and
unrealistically perpendicular (but maybe fragile to bend).

Look at real usage of glasses on tv or in pictures.  Many are
perpetually looking thru lenses at almost 45 degree angle, and I mean
for long views which excludes the bifocal issue.  Sometimes this is
exacerbated by habits of tilting the head back, or having a wide or
high nosebridge that jacks the frame up higher than it was designed for
(eg asia, africa)...