This is wrong - The body isn't a closed system, but an ongoing exporter of entrophy. There is no fundamental reason why "better repair mechanisms" wouldn't result in an permanent health. I don't like calling this immortality, because.. well, mishap and violence will still get you eventually, but the whole decay and slow dying thing isn't written into the laws of physics or even biology. It's just that Azathoth never had a reason to fix it.