Lingvogeometry

[Monolens]

So no, we don't get the word 'crone' from the curved shape of a stereotype's spine.

Well, I did not say we get it etymologically. However, there is a geometric similarity. One of noticeable difference of old people, especially crones, is the structure of their spine.
Etymologically “crone” comes from “meat” and “cut”.

crone (n.)
late 14c., from Anglo-French carogne, from Old North French carogne, term of abuse for a cantankerous or withered woman, literally "carrion," from Vulgar Latin *caronia (see carrion).
carrion (n.)
early 13c., carione, from Anglo-French carogne (Old North French caroigne; Old French charogne, 12c., "carrion, corpse," Modern French charogne), from Vulgar Latin *caronia "carcass" (source of Italian carogna, Spanish carroña "carrion"), from Latin caro "meat" (see carnage).
carnage (n.)
c.1600, from Middle French carnage (16c.), from Old Italian carnaggio "slaughter, murder," from Medieval Latin carnaticum "flesh," from Latin carnaticum "slaughter of animals," from carnem (nominative caro) "flesh," originally "a piece of flesh," from PIE root *(s)ker- (1) "to cut"

And couldn't all those blades be curved because that's a more efficient shape for slicing meat or whatever?

I did not say that they made their blade this shape because of the moon. Nevertheless, they recognized the oldest and powerful available symbol when they made technically effective shape for slicing meat. They get curved knife. Its shape was reminding them well-known object, which they met every night. It was associated with the moon thus.