Actually this is what you said:
You said the "natural course" after a broken leg is death. Which isn't true since animals have been shown to survive broken legs in the wild without any medical care. In fact you admitted it yourself that juveniles can survive broken legs because their younger bodies can be more readily repaired as well as the aid they get from their parents.
The broken leg isn't what kills animals. It's the lack of ability to take care of themselves while the broken bones heal naturally.