why are grandma's swag hooks busted?

Posted this on reddit:

"Seems like every house built in the 70s/80s has a bunch of these hooks in the ceiling and at least half of them are busted off in exactly the same place.I wondered if people were just hanging too much on them or if it's just bad design...so I ran the FEA.

Turns out, it's a fundamental flaw in the design.

Even at 25 lb load the red area on the hook sees stresses about 2x the yield strength of the cheap cast iron these are made of.A tiny localized peak stress in cast iron is a fatal flaw because it's so susceptible to crack propagation and that's why your grandma has half a dozen broken macrame hangers sticking out of the ceiling!"

Anyway, u/Derpygoras issued a challenge, I accepted and here's the result!

This is faaaar from optimized (look at all that freeloading blue wasted mass) but just changing the internal radius increased the load 3x!